The proximate cause of the outbreak of World War I was the assassination of the Archduke Franz Ferdinand of Austro-Hungary by a Serbian terrorist. And yet, by the close of the conflict in 1918, most people on the Allied side attributed sole blame for the War to Germany.
This belief was explicitly set out in the Treaty of Versailles, under which Germany was expected to pay huge financial reparations. These specific terms of the agreement were highly controversial in Germany. Most Germans believed that it was fundamentally unfair that their country should shoulder the blame for the war.
Despite the ostensible unfairness of the Versailles Treaty, however, one could reasonably argue that it was the ambition of Kaiser Wilhelm II that was the main reason why Europe went to war in 1914. The Kaiser's overriding ambition was to turn Germany into a world power. Jealous and resentful of Great Britain's status as an imperial power, Wilhelm believed that there was no reason why Germany couldn't also have its place in the sun, so to speak.
To that end, he embarked upon a rapid expansion of Germany's military capabilities, most notably in relation to naval power. The German Naval Laws passed between 1898 and 1912 were expressly designed to make Germany able to compete with Great Britain, at that time the foremost maritime power in Europe. The Kaiser believed that, with increased naval power, Germany would be able to advance its strategic interests more forcefully, not just in Europe but all across the globe.
Inevitably, the Kaiser's ambitions had serious consequences for European politics. In particular, they severely disrupted the balance of power in Europe. With Germany becoming militarily stronger and more aggressive in asserting its interests, other countries in Europe such as Great Britain began to see her as a threat. Where previously the focus of British foreign policy had been on deterring Russia, Great Britain now embarked upon a modernization of its navy as a response to increased German aggression.
To be sure, it wasn't inevitable that German ambitions to be an international power would lead directly to war in Europe. But it's reasonable to conclude that they were the most important single factor in its eventual outbreak. By 1914, a balance of power that had lasted for the better part of fifty years had been upended, making it much more likely that the European powers would settle their differences by force rather than diplomacy.