The Start of the War

World War I began on July 28, 1914, when Austria-Hungary declared war on Serbia. This seemingly small conflict between two countries spread rapidly: soon, Germany, Russia, Great Britain, and France were all drawn into the war, largely because they were involved in treaties that obligated them to defend certain other nations. Western and eastern fronts quickly opened along the borders of Germany and Austria-Hungary.
The Western and Eastern Fronts

The first month of combat consisted of bold attacks and rapid troop movements on both fronts. In the west, Germany attacked first Belgium and then France. In the east, Russia attacked both Germany and Austria-Hungary. In the south, Austria-Hungary attacked Serbia. Following the Battle of the Marne (September 5–9, 1914), the western front became entrenched in central France and remained that way for the rest of the war. The fronts in the east also gradually locked into place.
The Ottoman Empire

Late in 1914, the Ottoman Empire was brought into the fray as well, after Germany tricked Russia into thinking that Turkey had attacked it. As a result, much of 1915 was dominated by Allied actions against the Ottomans in the Mediterranean. First, Britain and France launched a failed attack on the Dardanelles. This campaign was followed by the British invasion of the Gallipoli Peninsula. Britain also launched a separate campaign against the Turks in Mesopotamia. Although the British had some successes in Mesopotamia, the Gallipoli campaign and the attacks on the Dardanelles resulted in British defeats.
Trench Warfare

The middle part of the war, 1916 and 1917, was dominated by continued trench warfare in both the east and the west. Soldiers fought from dug-in positions, striking at each other with machine guns, heavy artillery, and chemical weapons. Though soldiers died by the millions in brutal