World War I was a disastrous European war fought from 1914 to 1919 that also entangled many non-European areas, including colonial empires and the United States. The major powers included Britain, France and Russia on one side, and Germany, Austria-Hungary and Turkey on the other.
World War I started after Austria-Hungary invaded Serbia in retaliation for the assassination of Austrian Archduke Franz Ferdinand. This invasion created Europe-wide war by triggering a chain of pre-war alliances; Serbia allied with Russia, which was allied with Britain and France. Austria-Hungary was allied with Germany.
Germany struck into France through Belgium, hoping to eliminate France quickly before turning attention to Russia. However, armies quickly became bogged down due to technological advances in weaponry, such as modern artillery and machine guns. The war became one of attrition, as soldiers barely moved from their fortified trench systems despite endless artillery bombardments and atrocious losses on both sides. Approximately 10 millions soldiers were killed.
Prospects improved for Germany in 1917 when Russia collapsed into civil war, resulting in the eventual rise of the USSR. However, the United States also entered the war that year, helping push back German advances and leading to Germany's defeat.
Signed in June 1919, the Versailles Treaty to end the war significantly changed the borders of Europe, especially in the east. Germany lost territory and Austria-Hungary was split into pieces. The unstable condition of these new European countries, along with restrictions placed on Germany, set the stage for World War II.