A secret telegram to German minister in Mexico from German Foreign Secretary, Arthur Zimmerman,  is intercepted by British Naval Intelligence.  The telegram offers support if Mexico were to attack the USA.

British naval blockade causing food shortages, and starvation in Germany and Austria-Hungary. The average daily diet of 1,000 calories was insufficient to maintain a good standard of health, resulting by 1917 in widespread disorders caused by malnutrition such as scurvy, tuberculosis, and dysentery.

German submarine attacks on British shipping carrying food supplies to Britain leads to food shortages. A public campaign with a clear message is launched to encourage people to eat less bread.

USA break off diplomatic relations with Germany due to the reintroduction of the German navy’s policy of unlimited submarine warfare. Germany resume submarine campaign as a result and US President Woodrow Wilson cautioned what might happen if Germany followed through on its threat to sink American ships without warning.

German forces facing the Somme withdraw some 25 miles to new defences known to the British as Hindenburg Line – the withdrawal continues until 5 April.

The Zimmerman telegram is made public in America with express approval of President Wilson. The German Empire proposed financial support and a military alliance with Mexico, in the event of the United States entering the war against Germany. Germany's intention was to keep America neutral but public opinion was turning to war.

Start of first Revolution in Russia. Tsar Nicholas II abdicates and Provisional Government assumes power.

WAAC (Women's Army Auxiliary Corps) enrols 57,000 women - 9,000 WAACs to be sent to France. The WAAC was organised into four units: cookery, mechanical, clerical and miscellaneous but there were strict rules governing the use of women, such as, any job given to a member of WAAC, had to result in a man being released for frontline duties.

Lenin arrives back in Russia.

USA declares war on Germany. Wilson cited Germany’s violation of its pledge to suspend unrestricted submarine warfare in the North Atlantic and the Mediterranean, as well as its attempts to entice Mexico into an alliance against the United States, as his reasons for declaring war.

Battle of Arras - British advance 3.5 miles on first day and Canadians seize Vimy Ridge. Fighting continues until 4 May – both sides sustain very heavy casualties.

Second Battle of Aisne –  The operation had been planned as a decisive blow to the Germans but by 20 April it was clear that this had not been achieved and after heavy casualties fighting ends on 20 April.

Outbreak of mutinies take place after the defeat and the heavy casualties sustained by the French at Chalons-sur-Marne – indiscipline continues until October. 40-60 captured mutineers are executed but reforms to improve the welfare of French troops are introduced to help restore morale.

Convoy system (merchant ships under naval protection) introduced by British to combat German submarine threat. Previously opposed to the idea, the Admiralty agreed to it after 874,140 tons of shipping were lost to U-boats in April 1917. The convoy system quickly reduced losses in convoy to 10% of those suffered by independent ships.

US Congress pass bill to recruit 500,00 men into war service. This was necessary as only 73,000 volunteers enlisted out of the initial 1 million target.

Imperial War Graves Commission established in London (Now Commonwealth War Graves Commission).

First daylight aeroplane raid on Britain by German bombers. Cloud over London diverted 21 bombers to  secondary targets of Folkestone and the nearby Army camp at Shorncliffe. The raid resulted in 113 dead and 285 injured.

Battle of Messines begins when allied troops seize the Messines Ridge south of Ypres – fighting continues until 14 June.

Daylight raid by 18 German bombers on London – 157 killed, 432 injured; including 16 children killed by a bomb falling on a primary school in Poplar. This was the deadliest air raid of the war and some years later in 1938, it was described by Air Commodore Lionel Charlton as "the beginning of a new epoch in the history of warfare."

Portuguese troops see action on the Western Front for first time.

Just a few months after declaring war on Germany the first American troops arrive in France. By the end of the war the US had mobilised over 4,000,000 military personnel.

Russian summer offensive against Germany begins. Directed by war minister Kerensky Russia suffers a series of defeats – fighting continues until 18 July.

Conscription Bill passed by Canadian parliament. Despite many Canadians volunteering to join the forces earlier in the war, enlistment virtually stopped when people heard of the trench conditions and number of casualties. Conscription was necessary to deliver the promised 500,000 by the end of 1917.

Daylight bombing by 22 German Gothas over London kills 57 people and leaves 193 injured. One hundred sorties were flown against the formation, resulting in one Gotha shot down and three damaged.

British Royal Family change name from Saxe-Coburg-Gotha to Windsor. Anti-German feeling was at a high and so under pressure from all round the Royal Family chose to permanently separate themselves from Germany by dropping all German titles and dignities.

Third battle of Ypres begins. Better known as Passchendaele, the fighting continues until 10 November. Both sides experience heavy losses. In his Memoirs of 1938, Lloyd George wrote, "Passchendaele was indeed one of the greatest disasters of the war ... No soldier of any intelligence now defends this senseless campaign ..."

Heavy rain across Ypres battle field for nearly one month – prevents any progress. Pope Benedict XV makes another appeal for peace but is largely ignored, only Austria-Hungary are interested.

Mutiny in the German High Seas Fleet at Wilhelmshaven when around 400 sailors left their ships and began to protest for an end to the war. Army officials persuaded them back to their ships where 75 of them were arrested and imprisoned. The ringleaders were subsequently tried, convicted and executed.

Previously minister of war and of the navy, Alexander Karensky becomes Prime Minister of Russia.

French launch fresh attack at Verdun – recapturing most of the ground lost during 1916.

Germany had switched to night raids as a measure of protection against the improved air defences over Britain. The first night raid by the Gothas took place on 3 September over Chatham killing 152 including 130 naval recruits whose dormitory received a direct hit. Encouraged by the lack of night defence, a raid on London was carried out the following night.

Battle of the Menin Road – British and ANZAC troops at Passchendaele begin stead advance after rains of a month finally stop but the ground conditions are wet and muddy.

Last of the German bomber raids on London takes place.

Second Battle of Passchendaele - British and Canadian troops begin final attack on the village.

Balfour Declaration issued by British Government in support of a Jewish homeland in Palestine.

Second Russian Revolution. Bolsheviks led by Lenin and Trotsky seize power and overthrow provision Government.

Third Battle of Ypres ends with the capture of Passchendaele village.

Georges Clemenceau becomes Prime Minister and Minister for War in France. A fierce of critic of previous leaders handling of the war, Clemenceau faces a challenge to build morale and to convince his people that victory is the only choice for France.

Battle of Cambrai – combined use of tanks, aircraft and artillery - major advance made on the Hindenburg Line. Some 476 tanks took part in the battle of which 179 were lost. The use of tanks meant that lines of barbed wire defences, previously a major hinderance to soldiers, did little to slow the tanks.

Cambrai – Germans launch major counter-attack recapturing almost all the ground lost of the previous ten days. The German counter-attack confirmed the effectiveness of artillery, trench mortars, and evolving stormtrooper tactics.

Jerusalem taken by British forces in Palestine – ends 673 years of rule by Turkey.

Bolshevik Government sign armistice with Germany – suspends hostilities on the Eastern Front and on December 22, peace negotiations begin at Brest-Litovsk.