World War I (1914–18): Causes Although the United States did not enter World War I until 1917, the outbreak of that war in 1914, and its underlying causes and consequences, deeply and immediately affected America's position both at home and abroad. In the debate on neutrality and later on peace aims, much was made of European secret diplomacy, which was rejected on the U.S. side of the Atlantic, of militarism and the escalating arms race before 1914, and of the impact of colonialism. Undoubtedly, all these factors contributed to the origins of the European catastrophe, but they do not explain why the war broke out when it did. This question can only be answered more precisely by looking at the political and military decision‐making processes in the last months, weeks, and days of peace in 1914.
