Selective Training and Service Act of 1940 - Wikipedia

On September 14, 1940, the US Congress passes first peacetime conscription bill (draft law). From the article:

"The draft began in October 1940, with the first men entering military service on November 18. By the early summer of 1941, President Franklin D. Roosevelt asked the U.S. Congress to extend the term of duty for the draftees beyond twelve months to a total of thirty months, plus any additional time that he could deem necessary for national security. On August 12, the United States House of Representatives approved the extension by a single vote.[4] As Under Secretary of the Army Karl R. Bendetsen said in an oral history interview, "Mr. Rayburn banged the gavel at a critical moment and declared the Bill had passed."[5] The Senate approved it by a wider margin, and Roosevelt signed the Service Extension Act of 1941 into law on August 18.

Many of the soldiers drafted in October 1940 threatened to desert once the original twelve months of their service was up. Many of these men painted the letters "O H I O" on the walls of their barracks in protest.[6] These letters were an acronym for "Over the hill in October", which meant that the men intended to desert upon the end of their twelve months of duty. Desertions did occur, but they were not widespread. Following the Japanese attack on Pearl Harbor, Hawaii, on December 7, 1941, millions of American men entered the United States military's ranks both by volunteering and by conscription.

After the United States entered World War II, amendments to the Selective Training and Service Act on December 20, 1941, made all men between the ages of 20 and 44 liable for military service, and required all men between the ages of 18 and 64 to register. The terminal point of service was extended to six months after the war. Another amendment signed on November 13, 1942, called the registered 18- and 19-year-olds into military service. From October 1940 until March 1947—when the wartime Selective Training and Service Act expired after extensions by Congress—over 10,000,000 men were inducted."