On the 28th of June 1914, Archduke Franz Ferdinand was in Sarajevo for a state visit, when he was shot and killed by the Serbian revolutionary Gavrilo Princip. Princip was caught and put on trial, but he never showed any remorse. He believed Serbia should be free from Austrian influence, in his own words: "I am a Yugoslav nationalist, aiming for the unification of all Yugoslavs, and I do not care what form of state, but it must be free from Austria."

Austria believed that Serbia should be punished for Princip's actions and, with the promise of unconditional support from Germany, mobilised its armies and declared war on Serbia on the 28th of July 1914. Tsar Nicholas II of Russia declared war a day later; in a little over a month the Austrian-Serbian conflict had become European-wide, and the First World War had begun.

After joining the Triple Alliance, Turkey established a blockade in the Bosporus and the Dardanelles, cutting off the supply lines between between the allies and Russia. The British government decided to lift this blockade with a short, strong attack, starting in February of 1915.

The plan was to occupy the peninsula of Gallipoli. The navy was to weaken the Turkish defence, but British forces were only given a few days to prepare. This led to grave losses: over 700 men were wounded or killed, and a third of British ships were lost. It became clear that the Turkish navy was too strong, and an attack on land was planned. This became an even bigger disaster; thousands of soldiers were killed or wounded, and the peninsula remained in Turkish hands. It would be months before the order to retreat would be given.

The last allied troops finally retreated on 8 January 1916. The battle, that should have been decided in 48 hours, lasted almost a year. Ultimately, 205,000 British soldiers were killed, wounded, or missing.

On the 3rd of August 1914, however, the German army violated the treaty and invaded Belgium on its way to France. The British Foreign Secretary, Edward Grey, headed to the House of Commons to make a case for war.

He stated that, according to the Treaty of London, Great Britain was obligated to help Belgium in the case of an invasion. This was not necessarily true, as the Treaty declared that the neutrality of Belgium must be protected; not that military help was obligatory.

Grey, however, acted like this was the case, in order to convince the British people that a declaration of war was morally and legally the right thing to do. He was successful in this regard; the British people united behind their leaders. When Herbert Kitchener, the War Secretary, started a recruitment campaign, over a million men volunteered to fight.

On 1 July, at precisely 7:30 am, commanders ordered the attack. The British troops 'went over the top', they climbed out of the trenches and moved in the direction of German lines. However, the German defence had not been weakened enough, which stopped British soldiers from reaching the German trenches. The Germans had set up their machine guns, and mowed down the approaching troops. On that first day of battle, 21,392 British soldiers were killed, and around 35,500 were wounded.

General Douglas Haig, the commander during the battle of the Somme, after learning about the massacre that had taken place, decided to continue the attacks. Ultimately, 420,000 British soldiers were killed, wounded, or missing.

In response to this blockade, Germany declared the waters around Great Britain and Ireland, as well as The Channel, a war zone. Germany declared all-out submarine war: all enemy merchant ships, without exception, would be torpedoed.

However, this submarine war did not just affect the merchant ships, and on 7 May 1915 a German submarine torpedoed the British passenger liner Lusitania. Almost 1200 passengers were killed, and the American government demanded that Germany stop the submarine war. Germany complied, and did not resume the submarine war until 1 February 1917.

An investigation was launched, and it became clear that, should Germany succeed in blocking the naval trade lines between Great Britain and the Commonwealth, it would be a disaster. It was decided that, in case of war with Germany, the British navy would establish a blockade on the North Sea. Even though this plan was a violation of the Declaration of London (1909), which protected the right of neutral convoy, it was deemed militarily necessary.

The plan was put into action on 3 November 1914, and the North Sea was declared a war zone.

Because of the Easter Rising, conscription was not applied to Ireland, although many Irishmen volunteered to fight.

Three groups, each with their own motives, participated in the Easter Rising: the Irish Republican Brotherhood (IRB), the Irish Volunteers, and the Irish Citizen Army. The IRB was a secret, revolutionary organisation. This group planned the rebellion; they felt that a heroic gesture was needed to reawaken the spirit of Irish nationalism, and were motivated by frustration and pessimism. They did not necessarily believe that the revolution would be successful, but to them, failure was preferable to inaction.

Most moderate Irish Volunteers opposed the idea of unprovoked rebellion, because they felt it had no realistic change of succes; they wanted to fight in self-defence. Their leader, Patrick Pearse, became the face of the Easter Rising, and his distinctive ideas came to be identified with the rebellion as a whole, which is where the idea or martyrdom comes from.

The third group to participate in the Easter Rising, the Irish Citizen Army, was a small, socialist paramilitary organisation. Their participation in the Rising was an attempt to reconcile Marxism with nationalism. They also had a strong belief that working-class solidarity would have prevented a world war.

The preparation for the Easter Rising had completely gone wrong, and by Easter Sunday it had become clear that a rising no longer had any chance of success. At this point, the rebels had little to lose, as they would have been arrested anyway, and decided to go through with their plans. The rebels occupied prominent buildings in the city centre of Dublin, and managed to hold their positions for six days. By the end of the rising, large parts of the city centre had been destroyed, with many civilian casualties.

It was a day of jubilation; people gathered in large groups, cheering and singing. It was described by some as "the greatest day in history".

Germany was held responsible for starting the war, as its promise of support to Austria had pushed Vienna towards war. The terms of the Treaty were negotiated by Great Britain, France, the United States, and Italy. These countries imposed harsh terms on Germany, forcing it to give up much of its territory. The German army was reduced, and Germany was forced to pay for war losses.

The Treaty of Versailles did receive criticism; the US Congress never ended up approving the Treaty and later negotiated their own peace with Germany. The Germans were angry about being considered the sole country responsible for the war, and the total sum of the reparations Germany was forced to pay was so great, that no one expected them to be able to. The last payment was only made in 2010.

However, president Woodrow Wilson remained of the opinion that the United States should stay out of the war. "He has kept us out of war" was his slogan for re-election in 1916.

After Germany torpedoed the British passenger ship Lusitania, killing almost 200 Americans, the US started to move closer to the allies in Europe. When, in February 1917, Germany announced that the submarine war would be resumed, the American government severed all diplomatic ties with the country. A few weeks later, a declaration of war followed.