You'll find that almost everybody inducted into the Armed Forces after 7 Dec 41 and TRAINED in CONUS for a year prior to deployment overseas was awarded this.
While that's technically true, many folks who were drafted or inducted later in the war never served a year in the U.S. For example, my grandfather was drafted in October, 1944, went through basic training, had a short furlough and was sent to Germany as an infantry replacement with the 86th Division in February, 1945. He earned the European-African-Middle Eastern Campaign Medal with a battle star for the Central Europe campaign. Because his division was one of the last to enter combat in Europe, it was sent home in June, 1945, given a couple of months of intensive training to prepare for the invasion of Japan and sent to the Pacific where he also earned the Asiatic-Pacific Campaign Medal. But he never had enough cumulative time in the U.S. before his discharge to rate the American Campaign Medal.
Also, keep in mind that the American theater was huge and encompassed everything from the Arctic Circle to the tip of South America. Thousands of soldiers and sailors qualified for overseas bars for months and sometimes years of foreign service in such places as the Caribbean, Panama and Brazil where they engaged in anti-submarine patrols and stayed on alert for possible Axis invasions, especially early in the war. Those guys earned their American Campaign medals much the same as a supply clerk in Hawaii earned the Asiatic-Pacific Campaign Medal or an aircraft mechanic in England earned the European-African-Middle Eastern Campaign Medal.
I have a magazine type booklet published for the Army in 1942 or 1943 that breaks down all of the dozens of countries around the world where American troops were serving and it is amazing to see all the places, especially in South America and West Africa, where you usually don't think about us having troops during the war.