World War I was a war like no other, and U.S. participation in this global conflict had a profound impact on those who fought and on the future of the nation. Jump to: Background Suggestions for Teachers Additional Resources Background The Outbreak of War in Europe and the Debate over U.S. Involvement War broke out in Europe in the summer of 1914, after months of international tension. The spark that ignited open hostilities was the assassination of Archduke Franz Ferdinand, the heir to the thro