Which event signaled the start of World War I? Russia invaded Serbia, which angered the people of Germany.
Austria-Hungary invaded Serbia, which led Russia to get involved.
Italy invaded Germany, which violated a treaty from the late 1800s.
Great Britain invaded France, which caused Europe to take sides.

The Great War of 1914-18 began in August 1914. The causes of this war have been debated by politicians and historians ever since. One of the few things that historians have been able to agree about is that the war was the result of many different complex factors working together. These factors meant that the situation in Europe in 1914 was very tense. This in turn allowed one crisis to spiral out of control and spark off a war that killed millions.

The war was fought between rival alliances of European powers:

In 1879, Germany and Austria-Hungary formed an alliance (the Dual Alliance) that gave them great strength in the centre of Europe.

In 1892, the French and the Russians formed their own strong alliance (the Dual Entente) that meant Germany now had an unfriendly power on each side.

Soon afterwards, Germany's most powerful soldier, General Schlieffen, drew up a plan that would allow Germany to beat France very quickly in any future war. This would then free most German troops to fight Russia in the east.

By the early 1900s, the alliances had developed. The Dual Alliance had become the Triple Alliance with Italy (although Italy stayed out of the war in 1914).

In 1907, Britain joined Russia and France to form the Triple Entente. Britain was much less committed to this alliance than Russia or France.

Explanation:

The advantage of these alliances was that it gave the great powers a sense of security. The downside was that if the powers stuck blindly to their alliances, then a small-scale local dispute involving one power might drag the other powers in and turn into a major war.

The experience of the early 1900s seemed to suggest this would not happen. For example:

In 1905 and 1911, there were disputes between the powers over colonies in North Africa.

In 1908, Austria-Hungary took over the province of Bosnia, which contained many Serbs. Serbia and her ally Russia were furious, but there was no war as a result.

In 1912-13, there were a series of wars in the Balkans. Serbia emerged from these wars as the main victor and appeared to be a possible threat to Austria-Hungary. Even so, there was no major war.

Unfortunately, this changed in 1914. The assassination of Archduke Franz Ferdinand was the trigger that set off the Great War. Franz Ferdinand was the heir to the throne of Austria-Hungary. He was assassinated on 28 June 1914 by terrorists from one of Austria-Hungary's rival powers, Serbia. Austria-Hungary blamed Serbia, even though the terrorists were not connected to the Serbian government. On 31 July 1914, Austria-Hungary declared war on Serbia.

At this point, the alliances came into play. Russia mobilised its army ready to help the Serbs against Austria-Hungary. Everyone knew that if Russia attacked Austria-Hungary, then the alliance system meant that Germany and France could be pulled into the war as well.

The alliance system did not mean that a European war had to happen. Some say that it could still have been avoided - especially if Britain could have made Germany hold back from helping Austria-Hungary. Germany decided to help Austria-Hungary and declared war on Russia (1 August) and France (3 August). The German army prepared to attack France through Belgium, just as General Schlieffen had planned.

d.prevent the spread of communism outside of eastern europe.