What was the key cause of the First World War?

Many factors caused the outbreak of the First World War in the summer 1914, such that historians cannot agree on a single prime cause. Despite this, the key factors can be identified as the long-term creation of a complex web of alliances in Europe, rising nationalism, and a short-term lack of communication between the Great Powers during the July Crisis. A catalyst to these trends was the volatility of regions under the lessening authority of declining empires, namely the Ottoman Empire and to a lesser extent Austria-Hungary.It could be suggested that of these the Alliance System was the most significant, as it created a framework in which any small conflict could rapidly escalate into continent-wide war. A.J.P. Taylor called it a 'rigid diplomatic framework' that could easily 'produce huge explosions', and James Joll also supports this argument. It cannot, however, be stated that the Alliance System was the defining factor, as without intense nationalistic sentiment in the Balkans the war may have been without its spark of Austro-Serbian hostility. Additionally, as John Keegan makes clear, communication between the Great Powers during the July Crisis was appalling. He argues that none of the European nations wanted the war that turned out to be the 'Great War', bringing attention to the fact that there was no communication between allies France and Russia regarding their opening actions in the war; Germany's Kaiser Wilhelm left to go on a cruise; and Britain left it unclear as to whether she would stand shoulder to shoulder with France or simply watch in splendid isolation. Hence, these factors can all be considered key causes of the First World War, though no single one can be considered its defining cause.