The First World War lasted for four years, and cost the lives of roughly 10 million people. The primary theaters of combat spanned large swaths of Western Europe, as well as the Dardenelles, eastern Germany, Poland, western Russia, Serbia, and the Mediterranean. Lesser conflicts were fought in Asia, Africa, and the Middle East, and naval combat took place in virtually all the waters surrounding the combatant nations.

While combat was relatively fluid and mobile in Eastern Europe, in the west, it quickly became bogged down in the infamous trench fighting which has become one of the most iconic aspects of the war. After an initial advance by primarily German troop, which rapidly moved through Belgium and northern France, almost reaching Paris, the Allied Powers were able to push the Germans back to Flanders. Only a few months into the conflict, both sides now found it impossible to gain ground, and therefore, began to dig trenches for protection from machine gun and artillery fire. Initially intended to be a temporary measure, the trench lines remained almost unchanged for the next three years.

The deadlock was only broken in the final months of the war, when the exhausted German army made a last effort to break through the Allied lines and reach Paris before the U.S. was able to put a significant number of troops into action in Europe. The push failed, and from that point on, the Germans steadily lost ground, finally suing for peace talks before the Allied advance could reach German soil.

The treaties which ended the war redrew the map of Europe, and are widely regarded as being a significant factor in the outbreak of the Second World War only a generation later. By the time it was over, the war had caused the collapse of four empires - The German, Austro-Hungarian, Russian, and Ottoman - and instigated the Russian Revolution, which led to the creation of the Soviet Union.

The war also contributed to the worst epidemic in living memory, the so called Spanish Flu, or influenza, which is believed to have begun in military bases in the U.S in late 1917 and early 1918. The widespread movement of soldiers into and out of the war zones helped to spread the virus more rapidly than had ever been possible before; the epidemic would go on to kill an estimated 50 million people around the globe, representing 3% to 5% of the global population, by the time it petered out in 1920.

Several of the nations of the defeated Central Powers were broken up into new countries, which would later play a role in the outbreak of WWII. Enormous population transfers also took place after the end of the fighting, causing substantial instability and further aggravating the situation.

Combatants included (among others):

Allied Powers:

France
British Empire
Russia
Serbia
Montenegro
Belgium
Japan
Italy
Portugal
Romania
United States
Greece

Central Powers:

Germany
Austria-Hungary
Ottoman Empire
Bulgaria