Europe plunges into war due to rising tensions between countries. Militarism led countries to rapidly increase their armed forces. Imperialism and competition for land in Africa caused tensions. Countries formed alliances that divided Europe into two sides. Nationalism in Austria-Hungary and Serbia further escalated tensions when the Archduke was assassinated, which sparked World War I. Several major battles ensued between the countries, including battles in Belgium, Marne, Verdun, and Somme, which impacted the course of the war.