The bombing of Pearl Harbor by Japan was the last straw for the U.S government and directly led to the official declaration of war on Japan followed by the rest of the Axis Powers. The United States slowly inched its way into the war due to FDR wanting to join the war while most of the country was still in support of isolationism. At first in the Neutrality Acts of 1935, 1936, and 1937 stated that when war was declared, trade of munitions with any country in the war was prohibited.
However at the start of the war the Neutrality Act of 1939 was passed which allowed the US to help the Allied Powers through a policy of cash-and-carry which allowed the trade of American war materials to their allies in the war. The next step in entering war was the initiation of a peacetime draft. The war effort in America started before news of the Holocaust reached the rest of the world. News of what was going on in Germany was starting to come out in 1942.
At first this information was withheld from the American public by the US government. This was brought to light by Randolph Paul in a report concerning the Treasury Department at the time the war was going on. Government officials went as far as attempting to hide information to the public concerning the mass murder of Jews in Germany and also using their machinery to prevent the rescue Jews (Doc L). Upon the news reaching the public, pressure for the Allies to end the Holocaust and rescue the Jews increased, however the US government resisted. Historical interpretations revealed in some cases, the US deliberately went out of their way to not help stop the murders when Allied bombers were flying missions just outside of the death camp in Auschwitz but claimed destroying the camp was unfeasible (Doc A). The condemnation of the event was largely verbal and published in newspapers at the time (Doc K), but not much action was taken to stop the mass murder until towards the end of the war.