More than twenty countries controlling territory on six continents would declare war between 1914 and 1918, making World War I (also known as the Great War) the first truly global conflict. Nations fought for a variety of reasons: fears of attack, competition for foreign investment and territory, lingering anti-imperial sentiments, national pride, political pressures at home, and a system of opposing alliances that had developed over the previous 35 years.

The belligerent nations formed two broad military alliances: Britain and France allied with Russia, known as the Triple Entente; Germany, Austria-Hungary, and Italy joined the Central powers. Each alliance had its own regional rivalries and conflicts, but no country wanted to fight alone if war broke out. So, many of the world’s major powers signed agreements, called treaties, that guaranteed them support from their allies if they were at war with each other.

In August 1914, Germany invaded Belgium and northern France. The resulting trench warfare transformed the landscape of northern Europe into a lunar mud-scape of shell craters, corpses, and wire entanglements. After a series of costly offensives, both sides realized that victory would be won by outlasting the other–in other words, by a war of attrition.

As the war dragged on, shortages of food, metal, and other essential supplies created severe hardship for most of the world’s people. Families were stricken with privation, while factories were converted to the production of weapons and ammunition. To save resources, many people grew “victory gardens” and joined scrap collection drives. Rationing was implemented to control the price of food, sugar, gasoline, shoes, and other items.