1914.

The Great Powers of Europe are divided into two rival alliances:

The Triple Entente: France, Britain and Russia, united by fear and suspicion of Germany, Europe's

new strongest power.

And the Triple Alliance: Germany, which fears encirclement by its rivals; Austro-Hungary,

clinging onto a fragile empire; and Italy, seeking gains at French expense.

The spark comes on 28th June, in the city of Sarajevo.

Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, is assassinated by a 19 year-old Slav

nationalist named Gavrilo Princip.

Austro-Hungary accuses its Balkan rival Serbia of having aided the assassin, and sends an

ultimatum, demanding humiliating concessions.

Serbia rejects the ultimatum, and Austro-Hungary declares war.

Within hours Austrian forces are shelling Belgrade.

The Russian Tsar, Nicholas II, feels honour-bound to defend Serbia, a fellow Slav nation, and

orders the Russian army to mobilise.

German Emperor Wilhelm II has promised his support to Austro-Hungary.

He and his generals see conflict with Russia as inevitable – and the sooner the better,

as Russian strength grows year on year.

Russian mobilisation is used to justify German mobilisation, followed by a declaration of

war on Russia.

Germany knows war with Russia means war with Russia's ally, France.

It has developed the Schlieffen Plan to meet this threat of a war on two fronts – first,

its armies will advance rapidly through neutral Belgium to encircle and destroy French armies

near Paris, and win a quick victory.

Then its forces can move east to deal with Russia, whose huge army will take much longer

to mobilise.

And so Germany declares war on France.

Six million men are now marching to war across Europe.

Italy, however, remains neutral.

The terms of the Triple Alliance don't bind it to join an offensive war.

The United States also declares its neutrality.

President Wilson and the American public have no desire to get entangled in Europe's war.

Britain is France's ally, but at first it's not clear if it will join the war against

Germany.

But when German troops invade Belgium, whose neutrality Britain has guaranteed, an ultimatum

is sent from London to Berlin demanding they withdraw.

It's ignored, and Britain declares war.

A British Expeditionary Force lands in France, while the German invasion is held up for crucial

days by Belgian resistance at the fortress-city of Liège.

German troops commit several massacres against Belgian civilians.

The atrocities are inflated by Allied propaganda, and help turn public opinion in neutral countries

against Germany.

France, unaware of Germany's great encircling attack, launches Plan XVII, an offensive into

German territory.

But in the Battle of the Frontiers they're driven back, with enormous losses on both

sides.

The British Expeditionary Force clashes with the German army at Mons.

But the British are heavily outnumbered, and soon join the French in retreat.

The Allies make their stand at the River Marne, 40 miles outside Paris.

Their desperate counterattack saves the city and drives the Germans back.

Both sides suffer a quarter of a million casualties.

'The Race to the Sea' begins, as both sides try to outflank each other to the north.

A series of clashes leads to the First Battle of Ypres, where the Allies desperately cling

on and prevent a German breakthrough.

There are more heavy losses on both sides.

The two armies then dig-in along the entire 350 mile front, seeking shelter from deadly

machinegun fire and artillery shells.

Trench warfare has begun.

British warships win the first naval battle of the war at Heligoland Bight, sinking three

German cruisers.

Britain has the most powerful navy in the world: 29 modern battleships to Germany's

19.

They now impose a naval blockade on Germany, preventing contraband goods, including food,

from reaching it by sea.

The aim is to bring Germany's economy to its knees and force it to surrender.

But a week later, the British cruiser HMS Pathfinder becomes the first victim in history

of a lethal new weapon - the submarine-launched torpedo.

German submarines, or U-boats, have a surface range of 9000 miles, and can attack undetected

from beneath the waves.

They herald a deadly new challenge to Britain's command of the seas.

On the Eastern Front, Russian armies invade East Prussia.

But they blunder into disaster at the Battle of Tannenberg, where General von Hindenburg

and his Chief of Staff Erich Ludendorff mastermind a brilliant German victory, taking 90,000

prisoners and destroying an entire Russian army.

The Russians contribute to their own defeat by transmitting uncoded wireless messages.

A second massive German victory at Masurian Lakes forces the Russians into retreat.

In just six weeks, the Russian army suffers nearly a third of a million casualties.

Meanwhile Austro-Hungary's invasion of Serbia suffers a humiliating reverse at the Battle

of Cer.

Austro-Hungary's offensive against Russia also ends in disaster and retreat, with the

loss of more than 300,000 men.

The fortress-town of Przemyśl is cut-off and besieged by the Russians.

The Germans are forced to come to the rescue, launching a diversionary attack towards Warsaw.

It leads to weeks of brutal, winter fighting around the Polish city of Łódź, but there

is no clear winner.

Meanwhile, the Turkish Ottoman Empire has joined the Central Powers, declaring war on

its old enemy, Russia.

Turkish warships bombard the Russian ports of Odessa and Sevastopol, while in the Caucasus,

Russian troops cross the Turkish frontier.

Beyond Europe, the war rages on the world's oceans and in far-flung European colonies.

German troops cross into British East Africa (modern Kenya) and occupy Taveta; while Allied

forces seize the German colony of Togoland (modern Togo).

But British forces invading German Cameroon are defeated at Garua and Nsanakong, while

a 3,000 strong force attacking German South-West Africa, modern Namibia, is captured at Sandfontein.

A month later, British landings at Tanga end in chaos and defeat at the hands of a much

smaller German force led by Colonel von Lettow-Vorbeck.

Cut-off from Germany, Lettow-Vorbeck goes on to wage a highly successful guerilla war

against the Allies, tying down huge numbers of troops.

In Asia, Japan honours its treaty with Britain and declares war on Germany.

Japanese forces go on to seize the German naval base at Tsingtao.

The German colonies of Samoa and New Guinea surrender to troops from New Zealand and Australia.

But in the Pacific, off the coast of Chile, German Admiral von Spee's powerful East Asia

squadron sinks two British cruisers at the Battle of Coronel.

Both ships are lost with all hands.

Five weeks later, he runs into a British naval task force at the Falkland Islands.

Four of the five German cruisers are sunk.

Von Spee goes down with his flagship.

While in the Middle East, British troops seize control of the Ottoman port of Basra, securing

access to the vital Persian oil that fuels the British fleet.

That winter, Austrian troops finally capture Belgrade, but the Serbs then counterattack

and drive them back once more.

The fighting in Serbia has already cost around 200,000 casualties on each side.

In the North Sea, German warships mount a hit-and-run raid against English coastal towns,

shelling Hartlepool, Whitby and Scarborough, and killing more than a hundred civilians.

On the Western Front, the French launch their first major offensive against the German lines:

but the First Battle of Champagne leads to small gains at a cost of 90,000 casualties.

While in the Caucasus, an Ottoman offensive through the mountains in midwinter ends in

disaster at Sarikamish.

Turkish casualties total 60,000, many frozen to death.

On the Western Front, that first Christmas is marked in some sectors by a short truce,

and games of football in No Man's Land, the killing zone between the trenches.