WWORLD WAR I HAD NO SINGLE MOST IMPORTANT CAUSE. Growing competition over colonies and world markets, a belligerent arms race, and a series of diplomatic crises sharpened international tensions. On the home front, new forms of populist nationalism strengthened people’s unquestioning belief in “my country right or wrong,” while ongoing domestic conflicts encouraged governments to pursue aggressive foreign policies in attempts to bolster national unity. All helped pave the road to war.