Stuck between the lavish mansions of the Gilded Age and the scandalous excess of the Roaring Twenties, the Progressive Era is an underappreciated passage in American history. Roughly covering the 1900s and 1910s, the period earned its rather grandiose name thanks to sweeping social reforms, democratic development and economic expansion that shaped much of the America we know today. But how progressive was the Progressive Era really? And who actually benefitted from the so-called progress?

From presidential assassinations to the world’s first global war, click through this gallery to see stunning images from America’s most underestimated era…

We're kicking off with the Progressive Era's least progressive aspect. The period between 1890 and 1920 is often called 'the nadir of American race relations', and the notorious 'Plessy v Ferguson' case of 1896 goes some way to showing why. In 1892, Homer Plessy broke Louisiana's Separate Car Act by sitting in a whites-only train carriage, in order to challenge the law in the Supreme Court. Instead of striking down this obvious affront to the 14th Amendment, the court backed the state, arguing that the carriages were "separate but equal". Though farcical to modern eyes, the ruling gave segregation a legal basis that endured into the 1960s. This 1896 photo shows an exercise class in a segregated school.

America never considered itself an imperial power in the mould of Britain or France, but through the 19th century it nevertheless built an empire, expanding not only into the territories of Native Americans but overseas as well. In 1898 the US fought a short war against Spain in support of Cuban independence, won a quick and brutal victory, and inherited the colonies of Puerto Rico, Guam and the Philippines. Among the US troops to land on Cuba were the so-called 'Rough Riders' – a volunteer cavalry brigade led by the charismatic future president, Theodore Roosevelt. He's pictured here (centre) with his soldiers at the Battle of San Juan Hill.

The US was equally content to annex territory without the smokescreen of military glory. Hawaii spent most of the 1800s being slowly throttled by Western encroachment, with its monarchy downgraded, its economy reshaped by foreign businessmen, and its native language gradually sidelined and then prohibited. Hawaii's last sovereign, Queen Lili'uokalani, tried to push back, and was promptly removed in a bloodless coup by American sugar magnates and US marines. It took another century – and Hawaiian statehood – for Congress to officially acknowledge "the illegal overthrow of the Kingdom of Hawaii". Shown here is the 1898 annexation ceremony outside the Iolani Palace in Honolulu.

The Progressive Era was preceded by the Gilded Age, which today summons images of tilted top hats and enormous East Coast estates. But for the vast majority of Americans it was an era of poverty, as a handful of robber barons ran up unfathomable fortunes by dominating burgeoning industries, often aided by rampant corruption and the exploitation of workforces with few legal protections. Big business was suddenly hundreds of times bigger than it had ever been before, and Progressive Era legislation aimed to stop it crushing American society under its weight. This photo was taken around the turn of the century, and shows impoverished child miners in West Virginia.

Poor old William McKinley. When Abraham Lincoln and John F Kennedy were assassinated they were instantly lionised, and both now occupy spots at the top of the presidential pantheon. McKinley instead joins 20th president James Garfield on the short but coincidental list of presidents who were inaugurated in March, assassinated six months later and then quickly overshadowed. He was shot by disgruntled anarchist Leon Czolgosz and replaced by his VP Theodore Roosevelt, who would go on to earn a spot on Mount Rushmore. This photo shows McKinley's coffin being transferred to a hearse after his state funeral in Washington DC.

A war hero, noted outdoorsman and future recipient of the Nobel Peace Prize, the 42-year-old Roosevelt remains the youngest president America has ever had, and he brought vigour and energy to the usually venerable office. A tireless crusader and advocate of "the strenuous life", he whipped up crowds with pointed fingers and shaken fists, and invited cowboys, prizefighters and explorers to visit the White House. His appetite for reform made him a clear figurehead for Progressive Era ideals, and he touted "a square deal" between capital and labour. The phrase became the slogan of his successful 1904 reelection campaign.

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Words like 'legislation' and 'regulation' don't tend to thrill school history classes, but, as policy packages go, Roosevelt's Square Deal was pretty exciting. He revived a dormant 1890 law to forcibly break up the colossal corporate monopolies that dominated the Gilded Age, initiating lawsuits against 44 big businesses. During a 1902 mining strike he personally pressured the mine owners to negotiate a pay deal (he's pictured here with the miners shortly after). And he set aside vast areas as national forests, protecting five times as much land as all his predecessors combined. Historians today talk about the 'three Cs' of the Square Deal: consumer protection, corporate regulation and conservation.

One of the most famous photos in US history, this image captures the moment the Wright Brothers – Orville and Wilbur – achieved the world's first sustained and controlled flight with their prototype biplane, the Kitty Hawk. Orville is at the wheel and Wilbur is running alongside, easily keeping pace thanks to a 25 mile-per-hour (40km/h) headwind. The craft remained airborne for 12 seconds – enough to travel 120 feet (37m) and assure both brothers everlasting fame. Flying as an industry only really took off in the 1920s, as America's first airfields began to open along the East Coast. But it was here, a few feet above an unremarkable field in North Carolina, that modern aviation really began.

Immigration has been a key part of the United States' story since long before it was the United States, but even by American standards it was a hot topic in the early 19th century. Between 1900 and 1915 more than 15 million immigrants landed on US shores – roughly the same number as the previous 40 years. Many of these new arrivals came from non-English-speaking countries like Italy, Poland and Russia, and had a harder time adjusting to American life, while rising xenophobia saw tough new immigration laws introduced in the 1920s. This photo shows an official attaching labels to a German family at the Ellis Island immigrant processing station in New York, around 1904.

Today 83% of Americans live in urban areas, but in the Progressive Era the rapid growth of cities was a new and transformative trend. Cities meant opportunity; factories flourished, incomes rose and hordes of hopefuls flooded in from the countryside and from abroad. City centres prospered, with baseball fields, department stores, theme parks and improved public transport – this photo shows New York's subway shortly before it opened in 1904. But urban squalor was never far away, and estimates now suggest that by 1904 one in three city-dwellers was at risk of starvation. The have-nots lived in appallingly filthy tenement blocks, and accounts of their destitution intensified calls for reform.

Poverty and corruption exist in every age, but in the Progressive Era they were harder to ignore. Muckrakers – so-called because they combed through the darker, dirtier corners of society – became famous for publicising wrongdoing in America's institutions, and some of their names endure. Ida Tarbell (pictured here at her desk) wrote The History of the Standard Oil Company – a detailed 1904 expose on Standard Oil's predatory practices that helped spur the destruction of the company. Where Tarbell used a pen, Lewis Hine used a camera, snapping shots of impoverished child labourers to alert the public. Several of his photos are in this gallery.

Most muckrakers would now be called investigative journalists, but some found fiction to be a more meaningful medium for exploring society's ills. Upton Sinclair used his experiences working incognito in Chicago slaughterhouses to write his 1906 novel The Jungle, which turned heads and stomachs with its revolting descriptions of the unsanitary practices in meat-packing facilities. Roosevelt himself read and digested the book, before promptly passing the Pure Food and Drug Act and the Meat Inspection Act, which banned the sale of misbranded or adulterated foods and regulated the slaughter of livestock. Pictured here is a clam seller in New York around 1906 – the year both acts became law.

This devastating photo shows residents looking on as downtown San Francisco burns to the ground. The 1906 San Francisco earthquake remains one of the deadliest disasters in US history, with more than 3,000 dead and 80% of the city destroyed. Survivors recalled a noise "like the roar of 10,000 lions" and "a lot of people praying...thinking the end of the world was here". Broken gas lines sparked fires that burned for three days, and the prevailing images of the quake are of entire neighbourhoods disappearing into the smoke. Survivors watched the homes below them disintegrate one by one, before evacuating themselves as the flames advanced up the city's hills.

It is perhaps ironic that the 1920s – notoriously an era of excess – was the only officially booze-less decade in US history. Many Progressive Era reforms earn approving nods from historians – labour rights, consumer protections, building codes etc – but the push to ban alcohol tends to go down less well. At the time it was a natural Progressive cause: it was concerned with public morality, took aim at special interests and had wide support among the middle class. Groups like the Anti-Saloon League and the Temperance Movement saw alcohol as a social menace, and a blot on the nation's spiritual and physical health. This photo shows a saloon in Chicago, around 1908.

It may seem hard to believe now, but in the early 1900s income tax did not exist in the United States. Government revenue came mainly from tariffs, and the new tax was introduced under Roosevelt's successors: William H Taft (left) and Woodrow Wilson (right). Taft proposed it in 1909 as part of a broader tax reform package, and despite conservative opposition it was eventually passed by state legislatures and signed into law by Wilson in 1913. A high-minded idealist, Wilson was another titan of the Progressive Era, whose 'New Freedom' economic programme continued to break up monopolies, regulate banks and protect small businesses.

These are the earliest photos ever taken of America

The Gilded Age was a golden age for industry (between 1863 and 1899 manufacturing production rose more than 800%), but while robber barons like Cornelius Vanderbilt and JP Morgan became some of history’s richest men, 40% of industrial labourers earned below the poverty line. Workers' rights were a pivotal Progressive cause, particularly calls for an eight-hour workday, wage growth and restrictions on child labour (pictured here is an eight-year-old boy at work in a cotton mill in 1910). The National Child Labor Committee, formed in 1904, lobbied hard to restrict underage employment, but faced resistance from farmers and poor immigrant families who needed the extra income.

America's early factory workers toiled in dark and often dangerous conditions, and in 1911 a terrible tragedy put safety squarely in the spotlight. This image shows the aftermath of the Shirtwaist Triangle Fire – a deadly industrial disaster that unfolded in the streets of Lower Manhattan. On 25 March an overcrowded clothing factory, whose doors had been locked to prevent theft, burned to cinders along with 146 trapped employees – mostly immigrant women, some as young as 14. New Yorkers could only watch in horror, and the ensuing uproar led to the creation of the Factory Investigating Commission. It recommended more than 30 new health and safety laws, including fire codes.

President Roosevelt broke up many of America's most egregious monopolies – a process known as 'trust-busting' – and the dominoes kept falling when he left office. Founded by John D Rockefeller, Standard Oil was perhaps America's most hated company, described by The New York World as "commercial bandits...there has been no outrage too colossal, no petty meanness too contemptible for these freebooters to engage in". After five years in court, federal action finally won out in 1911, forcibly dividing the company into 34 separate businesses. Rockefeller himself was too big to fail, and thumbed his nose at the ruling by becoming the world's first billionaire in 1916.

This unobtrusive-looking photo tells a tragic tale. The gentleman on the left is James de Witt Seligman, while the lady on the right is his sister, Florette Guggenheim. They're standing outside White Star Line's offices in New York on 16 April 1912, anxiously awaiting news of Florette's husband, Benjamin, who had set sail from France on the RMS Titanic six days before. The supposedly indestructible liner struck an iceberg and sank in the early hours of 15 April, and only around 700 of its 2,200 passengers and crew made it back to land. Benjamin Guggenheim, unfortunately, was not among them.

Roosevelt made a sensational comeback in the 1912 election, standing unsuccessfully against both Taft and Wilson under the banner of the all-new Progressive Party. He was promptly shot by mentally ill tavern owner John Schrank while campaigning in Milwaukee. The bullet lodged in Roosevelt's chest, but was cushioned by his steel glasses case and a 50-page-thick copy of his speech entitled Progressive Cause Greater Than Any Individual. A seasoned hunter, Roosevelt correctly assessed that the bullet had not reached his lung and delivered his speech as planned, quipping to the crowd: "I have just been shot – but it takes more than that to kill a bull moose."

More than 50 years after America's first women's rights convention in New York, the Progressive Era saw the fight for women's suffrage finally inching towards its rightful, inevitable conclusion. In 1912, Roosevelt's Progressive Party became the first national party to support votes for women, but it wasn't just the vote that women were fighting for. Between 1880 and 1910, the number of women in employment increased from 2.6 million to 7.8 million, while legislation guaranteed the right to control their earnings, own property and take custody of their children. Women were now in the workplace, in the law courts and on bank statements. It surely wouldn't be long before they were in the polling booth.

It's a common misconception that Henry Ford invented the assembly line – but he certainly popularised it. Inspired by Chicago's meat-packing houses and the conveyor belts used in flour mills, Ford's first assembly line in his Highland Park factory in Detroit (pictured), broke down the car construction process into 84 distinct steps. Ford's Model T car previously took 12 hours to piece together; now it took one hour and 33 minutes, while low-skilled workers could perform repetitive tasks at specific points along the line. "When I'm through," Ford famously quipped, "just about everyone will have one" – a conviction supported by the Model T's astonishing sales figures over the next 15 years.

When the US began building the Panama Canal in 1904, it did so despite a chilling cautionary tale. In the 1880s the French spent nine years attempting the project, only to declare bankruptcy amid crippling design failures, widespread bribery allegations and the deaths of a horrifying 20,000 workers from tropical diseases. But America succeeded where Europe failed – connecting the Atlantic and Pacific oceans and saving ships five months and 8,000 miles (12,875km) of travel. This image shows a steam shovel clearing debris in 1913. The canal opened the following year – an engineering miracle of historic proportions, and an equally historic US foreign policy success.

DW Griffith's The Birth of a Nation earned a storm of controversy for its racist depictions even in 1915, so you can imagine how it looks today. The film glorifies and mythologises the Ku Klux Klan – a white supremacist hate group founded after the Civil War – and has been described by The Washington Post as "the most racist movie ever made". Unfortunately, it quickly became America's highest-grossing film to date, and contributed directly to the Klan's revival. Officially refounded in 1915, the Klan had 100,000 members by 1920, and between two and five million by the middle of the decade. This disturbing image shows the Klan inducting new members in 1915.

Margaret Sanger was born in 1879, and at age 19 she watched her mother die of tuberculosis, her body broken by the strain of 11 childbirths and seven miscarriages. In October 1916, Sanger, now a nurse and campaigner, secretly opened America's first birth control clinic in Brooklyn. This photo was taken shortly after the clinic opened – and shortly before it closed. Even giving advice about contraceptives was "obscene" under the repressive Comstock Laws, and after just 10 days Sanger was arrested and thrown in jail. She would have the last laugh, living just long enough to help licence the first contraceptive pill in 1960, and see the undoing of the Comstock Laws.

When the First World War broke out in 1914, America remained strictly neutral. Nearly two-thirds of Americans had direct heritage on one side or other of the conflict, and President Wilson's 'He Kept Us Out Of War' slogan won him reelection in 1916. But behind the scenes the US was lending vast sums to the Allied powers, and the sinking of the Lusitania by a German U-boat – sending 128 Americans to the bottom of the Atlantic – outraged the US public. When an intercepted German telegram offered Mexico US territory in return for support, America had seen enough. This extraordinary 1917 image shows recruits at the Great Lakes Naval Training Station in Illinois spelling out the word 'victory'.

The US declared war in April 1917, kickstarting a mass mobilisation that would see around four million men conscripted, trained and shipped off to fight. When they arrived in Europe these men found a war unlike any America had known – with trenches, tanks, bomber planes, machine guns and poison gas. More than 50,000 US soldiers were killed in action and more than 200,000 were wounded – while many more lost their lives to the deadly 1918 Spanish flu pandemic which killed an estimated 50 million worldwide. This photo shows American troops being taught to use a gas mask before being sent off to France.

America emerged from the First World War bloodied but unbowed, unlike the great powers of Europe, who emerged shattered, bereaved and deeply in debt – mostly to America. US farming and manufacturing boomed during the conflict, while America's winning hand earned its leaders a spot at the top table in the post-war negotiations – and in the new world order. Some historians argue that, despite the terrible human cost, the First World War paved the way for American superpower status. This photo shows victory celebrations on 42nd Street in New York on the day of Germany's official surrender – 11 November 1918.

The end of the war brought new appetite for change – particularly among women who'd stepped up and taken jobs in factories to replace conscripted men. Women had been voting in some Western states for more than 20 years, but it was only in the late 1910s that a campaigning blitz finally put a suffrage bill through Congress. This photo shows a jubilant suffrage worker surrounded by newspaper clippings reporting the passing of the 19th Amendment through the Senate, but the battle was not quite over. The bill still had to be ratified by 36 of the 48 states, which it was – just – thanks to a Tennessee delegate's mother scolding him before he cast his vote.

A month before the Senate passed the 19th Amendment, it had passed the Volstead Act – overriding a veto from President Wilson. Known as Prohibition, the act banned "the manufacture, sale or transportation of intoxicating liquors", starting in January 1920. Herbert Hoover called it "a noble experiment", but the determination of Americans to keep drinking resulted in a rise in organised crime more than a fall in anti-social behaviour. As the new decade dawned, Prohibition and women's liberation laid the groundwork for the flappers and gangsters still seen at costume parties today, and the beginning of another era in American history: the Roaring Twenties.

Now see the most extraordinary surviving images of America in the 1920s