In 1916, 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 1917, the strategy will push Europe's major powers to the brink of collapse.

1 February - 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.

3 February - 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.

19 February - 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.

24 February - In the Middle East, British forces avenge their 1916 humiliation at Kut, by defeating the Ottoman Turks and marching on to occupy Baghdad.

15 March - 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.

President Wilson announces the declaration of war to Congress, 1917.

President Wilson announces the declaration of war to Congress, 1917.

6 April - 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 886,000 tons of Allied shipping, an average of 17 ships a day, all packed with urgently-needed food and supplies. Britain will face starvation if the U-boats are not defeated soon.

9 April - 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 150,000 Allied casualties, to 130,000 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'. 

16 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.

17 April - British forces in Egypt advance across the Sinai Desert, but are thrown back by Ottoman forces at the Second Battle of Gaza.

7 June - That summer, at Messines Ridge, the British tunnel under the German lines, and detonate 19 enormous mines under the enemy position. Its the largest man-made explosion in history to date, and paves the way to a brilliant but highly local British victory.

27 June - In Greece, King Constantine, who has favoured neutrality, is forced to abdicate, and Greece joins the Allies.

1 July - Russia's Provisional Government orders a new attack, but the July Offensive is a disaster: the morale and discipline of the Russian army has collapsed. It can no longer be relied on to fight, and the Central Powers' counterattack is almost unopposed.

2 July - At sea, the Allies begin to group their merchant ships into convoys, which sail under naval escort. The new system leads to a steady fall in losses. The tide is turning in the U-boat war.

6 July - Arab rebels capture the strategic Ottoman port of Aqaba. They are accompanied by a British military advisor, Captain T.E. Lawrence, better known as 'Lawrence of Arabia'.

9 July - As discontent with the war grows in Germany, the German parliament, the Reichstag, passes a 'Peace Resolution', calling for 'a peace of understanding and reconciliation'. It's ignored by the German High Command, which now effectively rules the country as a military dictatorship.

An Allied naval convoy..

An Allied naval convoy..

22 Jul - Siam declares war on Germany, seeking to curry favour with the Allies. 

31 July - In Belgium, the British launch their major offensive of 1917, the Third Battle of Ypres. It will be remembered as Passchendaele. Heavy shelling, rain and broken irrigation channels turn the battlefield into a sea of mud. In these impossible conditions, all hopes of a breakthrough soon fade. The attack is called off after 3 months, by which point the British have suffered 240,000 casualties, the Germans 200,000.

4 August - Liberia declares war on Germany, after its capital is shelled by a German U-boat, itself in retaliation for Liberian co-operation with the Allies in the U-boat war. 

Chinese labourers at work on the Western Front, supervised by white British troops. 

Chinese labourers at work on the Western Front, supervised by white British troops. 

14 August - China joins the war on the Allied side, hoping to be awarded former German colonial territory at the war's end. China will contribute many thousands of labourers, working for the Allies in Europe, the Middle East and Asia.

18 August - On the Italian Front, at the 11th Battle of the Isonzo, Italian and Austro-Hungarian forces batter each other into exhaustion. There are 150,000 Italian casualties, 100,000 Austro-Hungarian.

24 October - With Russian forces in disarray, Germany is able to move troops from the East to the Italian Front. At the Battle of Caporetto, they help to smash through the Italian army, advancing 70 miles and taking quarter of a million prisoners. British and French divisions, desperately needed on the Western Front, have to be redeployed to shore-up the line.

31 October - the British, after two defeats, finally win at the Third Battle of Gaza, clearing the way for an advance into Palestine.

2 November - British foreign secretary Arthur Balfour issues the 'Balfour Declaration', expressing support for the creation of a national home for the Jews in Palestine. The aim is to rally Jewish support for the Allies, but the declaration contradicts existing pledges to Arab leaders.

7 November - In Russia, a second revolution brings Lenin's Bolshevik Party to power. He is determined to end Russia's involvement in the war.

13 November - In France, Georges Clemenceau becomes Prime Minister. Nicknamed 'the Tiger', he promises total war, and total victory. But for the Allies in late 1917, final victory looks uncertain: Russia has stopped fighting; French armies are recovering from mutiny; the Italian front has almost collapsed. And American reinforcements still seem a long way off. For the time being, the British are the only effective Allied force in the field...

20 November - The British attack at Cambrai, with the first major tank assault in history. On the first day, nearly 400 tanks spearhead an advance of several miles through German defences. But then the tanks break down or are knocked out; the Germans rush in reinforcements, and the gains are lost.

6 December - Finland declares independence from Russia.

9 December - Rumania, isolated by the Russian collapse, signs an armistice with the Central Powers. Six days later, Russia also signs an armistice. The Allied Eastern Front is no more.

11 December - General Allenby leads British troops into Jerusalem, ending 400 years of Ottoman rule.

1917 has seen one major Allied power, Russia, knocked out of the war – but the arrival of a fresh, new ally, America. Germany knows only military victory can now save it from being overwhelmed by Allied resources, and begins planning one last, massive onslaught, for the spring of 1918.

British General Allenby enters Jerusalem.

British General Allenby enters Jerusalem.

©Toby Groom 2016