World War I (1914-1918) was the first major international conflict of the 20th century. It was an armed conflict between the Triple Entente: Great Britain, France, Serbia, and Imperial Russia (later joined by Italy, Greece, Portugal, Romania, and the United States) against the Triple Alliance: Germany, Italy, and Austria-Hungary (later joined by Ottoman Turkey and Bulgaria). It was caused by many factors such as imperialism, the growing militarism of Germany, the formation of alliances between countries with similar goals, and the nationalism in regions of the Balkans. The assassination of Archduke Franz Ferdinand in 1914 triggered the start of the war. This historical conflict had 3 stages. The first of rapid offensives in the Eastern and Western fronts until the hostilities stalled in a Trench War. Mechanized warfare was also a feature of this conflict, with the introduction of tanks, armored vehicles, planes, and submarines in the combats. The Russian revolution caused the exit of Russia from the conflict and the US entered the war stage, which changed the conditions in the table and started a second rapid offensive stage where the Triple Entente achieved the German surrender.