1. World War 1 began due to rising tensions between European powers competing for colonies and alliances that divided Europe into opposing factions. 2. Trench warfare led to massive casualties with little territorial gain as new technologies like machine guns and poison gas were developed but tactics had not caught up. 3. The United States entry into the war in 1917 on the side of the Allies helped break the stalemate and led to an Allied victory. The resulting Treaty of Versailles imposed harsh terms on Germany, sowing resentment.