What are some decisions that European leaders made that led to World War I?

A great confluence of events made World War One possible. Hundreds of books and dissertations have been written on the topic, but most historians agree that several decisions by Europe's political leaders in the years and months leading up to the outbreak of war contributed to that horrible conflagration. Below, I have outlined the most consequential of these decisions.

1. Secret Alliances: In their attempts to secure lasting strategic advantages, Germany and the Austria-Hungarian Empire agreed that if either of them were attacked, the other country would come to its defense, regardless of the circumstances. France and Britain made a similar pact, and separately, France made an alliance with Russia. Finally, Russians saw Serbia and all of the Balkans as a key to their strategic interests, and viewed Austrian aggression as a serious threat to Russian sovereignty. Unfortunately, the secretive nature of all these alliances prevented them from having a deterrent effect. Instead, these alliances simply magnified the scope and severity of any potential conflict.

2. German Militarism: There was no one main aggressor or instigator in World War One, but the 1871 Unification of Germany under the Prussian leadership of Otto von Bismarck led to a superstate (Germany) whose economic strength and territorial ambitions upset the balance of power that had existed on the continent for some time. Key German military leaders (like General Erich Ludendorff) disliked Britain's arrogance and assertion that it possessed a historical and even moral right to maintain military superiority on the continent.

Military and political leader, Paul von Hindenburg, was also a major proponent of modernizing and enlarging German's navy so that Germany could rival and perhaps even overtake Britain as Europe's pre-eminent power. This desire for military supremacy led the German military in the early 20th Century to adopt...

3. The Schlieffen Plan: Named after the Field Marshal Alfred von Schlieffen, this plan outlined how Germany would conduct a future war on the continent. The main contention of the plan was that Russia, due to its relative poverty, lack of infrastructure, and poor military readiness, would take a very long time to mobilize troops on Germany's eastern border. France, according to German thinking at the time, was the greater threat. 

However, German military planners believed that if they initially directed the vast majority of their forces to the western front, then they could overwhelm French forces guarding Paris and negotiate a quick surrender. German military leaders were correct in assuming that the French government would not have the stomach to allow Paris to be decimated in a siege. However, the Germans were surprised when advisors to Tzar Nicholas of Russia were able to mobilize Russian troops, who crossed into eastern Germany in a matter of a weeks.

This infamous miscalculation forced Germany to fight a war on two fronts, and virtually guaranteed a war of stagnation and attrition.