When the United States declared war on Germany on April 6, 1917, it had a regular Army of only 200,000 men. Needing a larger force, Congress passed a Selective Service Act that made all able-bodied men between the ages of 21 and 30 subject to military service. The government put nearly 4 million men in uniform and sent about 2 million soldiers overseas as members of the American Expeditionary Force. World War I infantrymen were called "Doughboys" because the buttons on their uniforms resembled a popular Civil War era donut called a "doughboy."