World War I began in 1914 with both sides expecting a quick victory but instead resulted in a bloody stalemate as the Western Front bogged down in trench warfare. By 1915, both sides had dug elaborate trench systems protected by barbed wire and machine guns. Planes were introduced to spot enemy positions and conduct aerial attacks. Neither side could gain a decisive advantage on land, so they sought new allies and opened new fronts, like the disastrous Gallipoli campaign. The U.S. entered the war in 1917 after German U-boats sank American ships. Total war mobilized entire populations and economies but took a heavy toll in casualties and civilian hardship.