Short answers, pulled from the story.
Europe first, also called Germany first, was the grand strategy agreed by the United States and the United Kingdom after the US entered World War II in December 1941. It called for using the greater share of Allied resources to defeat Nazi Germany while fighting a holding action against Japan in the Pacific.
The strategy was affirmed at the Arcadia Conference in December 1941, when President Franklin Roosevelt and Prime Minister Winston Churchill met in Washington shortly after the United States entered the war. Its foundations were laid earlier, in the ABC-1 conference report dated the 29th of March 1941.
Japan had attacked the United States directly at Pearl Harbor on the 7th of December 1941, and its navy threatened US territory in ways Germany could not match. As a result, American resources devoted to defeating Japan initially outpaced those sent to Europe, and not until 1944 did resources clearly favor Germany.
The strategy created friction between the Navy and the Army. Fleet Admiral Ernest King supported Europe first overall but pushed for a larger Pacific share, and his combined influence with General Douglas MacArthur helped increase the resources sent to the Pacific.
Public opinion favored fighting Japan first. A February 1943 poll found that 53 percent of Americans considered Japan the chief enemy compared with 34 percent who named Germany, and a later poll found 82 percent believed the Japanese were more cruel at heart than the Germans.
The strategy became a reality only in early 1944, when the buildup of American forces for the invasion of France shifted the balance of US resources toward Europe. By the war's end in Europe, the US Army had 47 divisions there compared with 21 Army divisions and 6 Marine Corps divisions in the Pacific.