The main reason why large-scale conflict broke out in World War I was the series of entangling alliances that existed before the war. In the pre-war years, all the nations of Europe were heavily nationalistic and heavily militarized. They all feared the strength of each other and sought to ensure their protection by signing treaties. So, the Germans, surrounded on all sides by possible enemies, signed alliances with Austria-Hungary and Italy. The French, fearing the threat posed by the Germans, signed treaties with Russia and Great Britain. The Triple Entente was signed between France, Britain, and Russia to provide for their mutual protection. 

The Triple Alliance, designed to counter the threat of the Triple Entente (the alliance between Britain, France, and Russia) involved an alliance between Germany, Austria-Hungary, and Italy.

The League of Nations was the brainchild of American President Woodrow Wilson who put forth the suggestion as part of his Fourteen Points at the Paris Peace Conference. Although the League was approved and formed by the international powers, when Wilson returned to America, he found that Congress was unwilling to immerse America so fully in the global experience. Congress rejected the peace treaty and American involvement in the League of Nations. This ensured that from the very beginning the League was without one of the preeminent world powers.

During the early twentieth century, Germany and France heavily disputed the status of Morocco. This was one of the primary sources of tension between the two nations in the buildup to World War One.

Gavrilo Princip assassinated Franz Ferdinand and his wife on July 28^th, 1914. Princip was a member of the Black Hand, an organization dedicated to the liberation of the Yugoslav people from the rule of the Austro-Hungarian empire. The Austrian empire used the assassination as a pretext to invade Serbia and thus committed the first act of aggression of the First World War.

The assassination of Austrian Archduke Franz Ferdinand by Serbian nationalist forces in 1914 was the direct inciting incident that led to the outbreak of World War One. In the years leading up to World War One the major powers of Europe signed extensive alliances with one another that ensured that if one country declared war on another country all the rest would be dragged into the conflict like falling dominoes. The Triple Entente between Britain, France, and Russia made the Germans and Austro-Hungarians feel threatened; the British promised to protect Belgian sovereignty; the Russians promised to fight for Serbian autonomy and so on.

Communism was not yet a major factor in international affairs in the period leading up to World War I, as the Russian Revolution had not yet happened. Nationalism and pan-slavism led to the increased ethnic tensions in Europe, especially in the Balkans, and they ultimately provided immediate catalysts for the war, namely the Serbian murder of Austrian archduke Franz Ferdinand and Russia's alliance with Serbia. Imperialism and militarism both contributed to growing rivalry and tension among the major European powers, namely Germany, Britain, and France. Imperialist ambitions, in particular, led to diplomatic incidents (such as the Agadir Crisis of 1911) that heightened tensions among these nations, especially due to British and French fears of German aggression.

The Schlieffen Plan began life as an experiment within the German military command at how the Germany army might effectively fight a simultaneous war against Russia in the East and France in the West. Following the outbreak of the First World War, the plan was implemented in part.

David Lloyd George assumed leadership of the British government in 1915 and ruled as Prime Minister until 1922. As such, he was the British representative at the Paris Peace Conference in 1919 where the victorious Allied powers discussed how to reorder Europe and the world in the wake of the First World War.

Although signed into an alliance with France and Russia, Britain originally wanted to remain primarily neutral in the First World War. Certainly, Britain did not want to fully commit its forces to the conflict. However, Britain was sworn to defend Belgian sovereignty. Once Germany invaded Belgium, Britain was forced into the war on the side of France and Russia against the Germans.