In the space of a few hours, the British suffer casualties, a third of them killed. It's the worst day in the history of the British army.
But more attacks are ordered, and the battle will rage for another five months.
Encouraged by the Russian advance, Romania joins the Allies.
But despite an initially successful advance into Transylvania, Romania quickly faces a counter-offensive from German, Bulgarian and Austro-Hungarian forces.
The Allied force at Salonika tries to support Romania, by launching their own offensive towards Monastir. With Serbian troops in the lead, there are small gains, but dogged Bulgarian resistance prevents a breakthrough.
On the Western Front, General von Falkenhayn finally calls off the attack at Verdun.
The French army has honoured their commander, General Nivelle's, promise – 'Ils ne passeront pas' – they shall not pass...
But victory comes at a terrible price: casualties.
The Germans lose almost as many. Verdun remains one of the bloodiest battles in human history.
For his defeat at Verdun, Falkenhayn is sacked, and Germany's heroes of
the Eastern Front, von Hindenburg and Ludendorff, take command in the west.
Meanwhile, the Battle of the Somme continues. Near the village of Flers,
the British introduce a new weapon they hope can break the deadlock of the trenches:
it is called the tank. But despite some small successes, the first tanks are are too few in number, and too prone to mechanical failure, to make any real impact.
On the Eastern Front, Russia's Brusilov Offensive comes to an end.
Casualty estimates vary wildly, but it's clear both sides have suffered catastrophic losses.
Neither the Russian nor the Austro-Hungarian army ever fully recovers.
On the Italian front, heavy fighting rages throughout the autumn, as Italian
forces make repeated, costly assaults against Austro-Hungarian positions along the Isonzo River.
The Battle of the Somme comes to an end amid autumn rain and mud. The Allies have advanced ten miles at the cost of casualties. German losses are about
The Allies reasssure themselves that this is a winning strategy,
because at this rate, Germany will run out of men first.
Meanwhile, disaster engulfs Romania, as the country is overrun by the Central
Powers. Romanian forces suffer a quarter of a million casualties.
The remnants of its army take position alongside the Russians on the Eastern Front.
That winter, Franz Josef, Emperor of Austria since dies. He is succeeded by his son Karl.
In Britain, Prime Minister Herbert Asquith is forced from office, and succeeded by David Lloyd George.
...While General Joffre is replaced as French commander-in-chief by General Nivelle, who promises victory through bold, aggressive action.
Amid the comings and goings, US President Woodrow Wilson's attempts to mediate a peace settlement come to nothing: neither side is willing to make concessions.
In World War One became a war of attrition.
Both sides began to focus less on winning victory on the battlefield, than grinding down the enemy, and inflicting such enormous losses they would be forced to surrender.
In the strategy will push Europe's major powers to the brink of collapse.
Germany knows it will lose a long war of attrition against the Allies, who have greater resources.
So its leaders gamble: they resume unrestricted submarine warfare, believing their U-boats can cut off Britain's food imports by sea, and starve the country into surrender within six months.
But the new shoot-on-sight tactics mean neutral American ships will inevitably be
caught in the crossfire, risking America joining the war on the Allied side.
Just two days into the campaign, the SS Housatonic, an American steamer carrying wheat from Galveston, Texas to England, is sunk by a U-boat.
The British then pass to the US government a telegram they've intercepted, from German
foreign secretary Arthur Zimmermann to the German ambassador in Mexico.
Germany is encouraging Mexico to attack America, if America and Germany end up at war.
The so-called Zimmermann Telegram puts
yet more pressure on US President Wilson to declare war on Germany.
In Russia, enormous casualties and bread shortages lead to riots... and revolution.
The Tsar abdicates. A Provisional Government takes charge, pledging
to continue the war. But at the front, Russian troops begin to desert en masse.
After a string of German provocations, the US finally declares war on Germany. It brings immense resources to the Allied cause, but they will take many months to mobilise.
And the German gamble of unrestricted submarine warfare may still pay off. April is the U-boats most successful month of the war: they sink tons of Allied shipping,
an average of ships a day, all packed with urgently-needed food and supplies.
Britain will face starvation if the U-boats are not defeated soon.
On the Western Front, the British launch the Battle of Arras - a diversion,
to support a major, upcoming French offensive.
After heavy fighting, Canadian troops seize the high ground of Vimy Ridge.
Its a limited Allied victory, but costs Allied casualties, to German.
Above the trenches, the first air war has reached new levels of sophistication and deadliness.
Reconnaissance aircraft are crucial for spotting enemy positions, and directing artillery fire onto them. Scout aircraft, or fighters, try to shoot them down before they can execute their mission.
New models of aircraft are developed every few months. But that spring, the superiority of German aircraft leads to heavy Allied losses, in what becomes known as 'Bloody April'.
Three days after the fall of Vimy Ridge,
French General Robert Nivelle launches his main offensive. Expectations are high,
but after initial success the advance bogs down and casualties quickly mount on both sides.
The apparently senseless losses cause morale in the French army to collapse.
Whole units mutiny, refusing to attack.
General Nivelle is sacked as French commander-in-chief, and replaced by
General Pétain, hero of Verdun, who promises no more suicidal attacks.
That summer, at Messines Ridge, the British tunnel under the German lines, and detonate enormous mines under the enemy position. It’s the largest man-made explosion in history to date,
and paves the way to a brilliant but highly local British victory.