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Archduke Franz Ferdinand of Austria is assissinated in Sarejevo (Bosnia-Herzegovina annexed by Austria-Hungary in 1908).

He is killed by a Bosnian Serb, Gavillo Princip, who was a Yugoslav nationalist. He worked together with a group of other nationalists, which were later known as 'Young Bosnia'. Ferdinand's wife, Sofie, Dutchess of Hohenberg, was also killed on that day.

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Austria-Hungary declares war on Serbia.

Russia feels obliged to back for Serbia, as Austria-Hungary had already taken in Serbia's capital city Belgrade. Russia partially mobilised its army, as Belgrade was only a few kilometres from the Russian borders. Now, Austria-Hungary and Germany (ally of Austria-Hungary), as a reaction of the Russian mobilisation, also mobilise their armies, and declare war on Russia. After this, there is a chain reaction of war declarations, as there were multiple allies, who felt obliged helping the other.

The Great powers of Europe were divided into two 'blocks': The Triple Entente, consisting of France, Russia and Britain, and The Triple Alliance of Austria-Hungary, Germany and later Italy.

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Germany invades Belgium and Luxembourg.

These invasions were part of 'the Schlieffen plan', a plan by Field Marshal Alfred von Schlieffen, which intended to invade France and Belgium, but mostly invading Paris as fast as possible. They did this because Germany was terrified of having to battle on two fronts; against France in the west, and against Russia on the east, due to the Franco-Russian Alliance formed in 1892. Not wanting to fight on two fronts, they came up with a plan to invade Paris as fast as possible, by going around the city, and surrounding it , and then play the defensive hand against Russia.

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First Battle of the Marne.

The Central powers had already invaded Belgium and Luxembourg, and some part of France, according to the Von Schlieffen plan, but then the 'Allied' powers fought back. This was a battle between 900.000 German soldiers, and 1.082.000 Allied soldiers, from which there were 64 French and 6 British batalions. In this battle 513.000 soldiers were injured, from which 148.700 ultimately died.

In the end the Allied powers did win, and made the German retreat further back.

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Battle of Verdun.

The battle of Verdun is the longest battle in the entire world war. It went on for more than 300 days. It was such a long battle, because both parties were lying in trenches. A lot of soldiers died from sickness, infections, hunger, cold etc. The Germans made use of the horrible mustard gas, which the allies knew nothing about, and made great casualties.
German forces lauched the offensive trying to force the French country to the negociating table, getting them to surrender.
But the French definitely didn't surrender. They fought with chauvinsm, wanting revenge for the Franco-Prussian war, which they lost.

In the battle, a million soldiers were injured, and 600.000 were killed. The numbers where somewhat equally divided between both parties.

The Germans made use of the horrible mustard gas, which the allies knew nothing about, and made great casualties.

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Naval Battle of Jutland.

This was the largest naval battle of WW1. It was a battle between the Royal Fleet of Britain, and The Imperial German fleet. It was fought in the North Sea near the coast of Denmark.
There were a total of 250 battleships involved, and around 30 ships were sunk, while also a lot of ships were very badly damaged. In the battle, around 9.000 people were killed, from which 6.000 were British, and a bit less than 3.000 Germans were killed.
After the battle, both sides claimed that they had won the battle, although in terms of casualties, it seemed like the Germans had won. The British navy though, stated that they were defending the blockade to the seas behind Britain, and that they succeeded in holding the Germans off.

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Battle of the Somme.

The battle was fought on both sides of the Somme. A total of 3 million men fought in the battle, and a total of 1 million were either wounded or, ultimately, killed.

The battle was part of the allied offensive, against the German Empire. It was a ferocious battle, and in the end the allied armies had won (only) 10 km of the German ground. The battle went on for 140 days, and was the first 'big win' since the battle of the Marne.
The battle of the Somme is seen as 'the most famous' battle of World War 1, as it was such a big victory for the allied powers.

The Battle of the Somme is seen as the beginning of modern all-arms warfare.

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Russian Revolution.

Russia had been suffering from a lot of social and economic problems, due to World War I. After some major losses due to battles, the Russian army revolted against its country. A group of soldiers and urban industrial proletariats was formed, who called themselves 'Sovjets', and they caused Tsar Nicholas II to step down his throne. The leader of one of these sub-groups of the Sovjets, the Bolsheviks, Vladimir Lenin, demanded Russia ceasing the fight with Germany which they refused (does happen eventually).
Later, a civil war had begun, between the 'Reds' (Bolsheviks) and the 'Whites' (Counter-Revolutionairies) which the 'Reds' eventually won after years of fighting, and called themselves the 'Communist Party' and later became the USSR as we know it.

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United States declares war on Germany.

The US had stayed neutral since the beginning of the war, which was already 3 years going, but had been supplying weapons and food to the United Kingdom and France and the others of the allied powers already.
The President at that time, Woodrow Wilson, made the choice to deal with foreign problems, although the country was at peace at the time, and didn't have to get involved with the war, but felt that they somewhat had to, because if Germany was going to win, they might be attacked as well.

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American combat forces arrive in France.

On this date, the first 14.000 U.S. infantry troops arrived at the port of Saint-Nazaire. They were welcomed by an enthusiastic crowd. 14.000 may not seem like a lot, but in the next weeks and months, a lot more arrived all across the coastline. In May 1918, over a million American troops were stationed in Europe, which is a lot for a country not in Europe.

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Treaties of Brest-Litovsk.

Russia had some serious losses against Germany, and had a few small victories against Austria-Hungary. Due to the enormous losses, and lots of casualties, the war was leading to extreme discontent among the entire population of Russia. After the Russian Revolution was started, and Lenin became the leader, one of the first things he did was putting a halt to the Russian participation to the war. A cease-fire was announced, but an actual agreement of peace was more difficult to reach.
In december 1917, discussions had already began: They couldn't find territorial agreement. A minister of Russia at the time, Leon Trotsky, said that the demands of territory was way too big.
Fighting resumed for a short period of time, and Russia, already weakened' knew that they had to give in to the Central's demands.
The Negociations resumed again on March 3rd, 1918, in Brest-Litovsk, and they came to a final treaty, and peace between the Central powers and Russia was found.

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Last German Offensive (Spring Offensive)

In the spring of 1918, the Germans did one desperate attempt to win the war, because the United States were deploying more and more soldiers, and they tried to defeat the French army before the deploying was fully done.
Ultimately, the allied powers, reinforced with soldiers from the U.S., was way too powerful for the already exhausted German armies. The total army of the Central powers was shrinking enormously, and the morale of winning this war was gone.
The offensive failed, and the armies were damaged hard.

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Final allied offensive.

After the failed Spring Offensive, the allied powers were quite sure of the victory. They went for a final offensive, also knowns as the 'Meuse-Argonne Offensive'. With the help of the armies of the U.S., the remaining Central armies were no match for the allies. The offensive lasted 47 days, when the 'Armistice of Compiège' was announced.

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Armistice of Compiège.

The German army  shrinked, and the morale was gone, leading to the soldiers not wanting to fight anymore. The other Central powers had already signed armistices, so Germany was the only 'enemy' left. They had no option to give up.

In total, World War I took 20 millions lives. Among them were soldiers, but also a lot of civilians. The majority of Europe thought one thing; 'Never again'.

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Treaty of Versailles.

In the treaty was stated, that the territories that had been won  had to be given back, and that reparations had to be paid for the damage done. Also, Germany wasn't allowed to have a big army anymore, and the Rhineland had to be demilitarised.

The treaty of Versailles is seen as one of the reasons for World War II, as Hitler used it for nationalising his country.