18 February
The German colony of Cameroon falls to French and British forces after 17 months of fighting.
21 February–19 December
The Battle of Verdun begins. Verdun, a French fortress town, was located in the western shadow of the Meuse river, close to the German-French border. It was of great symbolic significance to the French as it was the last town to fall in the Franco-Prussian War of 1871.  In the ten month battle, one of the longest in the war, both sides suffered almost one million casualties. The aim of the German army was to “bleed” the French army to death by capturing Verdun. The Germans launched six offensives between February and November. By October the French succeed in pushing back the Germans. Verdun is considered the bloodiest encounter in the war with the French suffering 540 000 and the Germans 430 000 casualties.
6 March
The second German offensive against the French at Verdun is launched.
18 March
Russians try and divert German troops from Verdun by staging an offensive on the Eastern Front in the area of Vilna in Lithuania. They are defeated by the Germans, suffering 70 000 casualties.
9 April
The beginning of the third German offensive at Verdun.
29 April
13 000 British and Indian soldiers surrender to the Turks in the five month siege of Kut-al-Amara in the Middle East. 
3 May
Germans launch fourth attack on Verdun.
25 May
Britain introduces conscription for all men between the ages of 19 and 40. Only those employed in agriculture, mining and railroads are exempted.
31 May
The Battle of Jutland, in the North Sea near Denmark, between the British and German naval fleet takes place. It ends in a stalemate with the Germans withdrawing and the British head home instead of pursuing them.
4 June
On the Eastern front, four Russian armies under General Alexei Brusilov launch an offensive along a 300 mile front defended by Austro-Hungarian troops. The Germans withdraw four divisions at Verdun to assist the Austro-Hungarian army.
22 June
The Germans renew the offensive at Verdun.
24 June
In preparation for the big Allied offensive that will become known as the Battle of the Somme, a weeklong artillery bombardment is launched by the Allies on the River Somme in Northern France. The aim of the bombardment is to destroy the German trench defence to allow for a rapid infantry advance on German frontlines on the first day of the planned Somme Offensive (to take place 1 July). Over 1.5 million shells are fired along a 15 mile front.
1 July 1916
The Battle of the Somme begins under the direction of British Commander in Chief, Sir Douglas Haig. It would last until November 19. On the first day, the British army suffers a death toll of 18 800 men. Total casualties after the first day number over 57 000.  
Total losses on the British side was 419 654 and on the German side between 450 000 and 680 000. The Battle of the Somme is regarded by many as one of the most colossal slaughter of men in military history.
15 July–20 July
The Battle of Delville Wood, part of the Somme Offensive takes place.  A South African Brigade of 3 150 men, attached to the 9th Scottish Division was instructed to “take the Wood and hold it at all costs”. The Brigade succeeded in capturing the wood after six days of fierce fighting and heavy artillery bombardments from the Germans. Only 750 men survived the battle.
27 August
Romania enters the war on the side of the Allies, and invades Austria-Hungary.
28 August
Italy declares war on Germany. Kaiser Wilhelm appoints Field Marshall Paul von Hindenburg as Germany’s new Chief of General Staff. Germany’s economy is placed under the Hindenburg Plan which grants the military new powers over the labour force, armaments production and the distribution of food.
15 September
Tanks are introduced for the first time in the Battle of the Somme by the British.
20 September
The Germans withdraw 24 divisions from the Western Front and combine them with the remainder of the Austro-Hungarian troops to meet the Russian offensive on the Eastern Front.  This new combined army force the Russians to withdraw from the areas they had won. The Russians lose nearly one million men and the withdrawal leads to a loss of morale in the Russian army.
18 November
Battle of the Somme ends.
7 December
Lloyd George becomes Prime Minister of Great Britain.
12 December
Joseph Joffre resigns as Commander in Chief of the French Army and is replaced by General Robert Nivelle
15 December
The French bring the Battle of Verdun to an end by launching an offensive that leads to the German withdrawal from Verdun.  The combined losses of both armies number almost one million after ten months of fighting at Verdun.
References

Collections in the Archives

World War I Timeline 1916
World War I