basically, an excuse for some people to hate Japanese. In World War II the Japanese military attacked the U.S. station in Pearl Harbor, Hawaii. This led to the U.S. entering the war. While people here can forgive Germany for what the atrocities the Nazi government did, they still hold a grudge against ALL Japanese people, despite the fact that General Hideki Tojo(the fascistdictator of Japan) and his cabinet were tried as war criminals and executed, and Japan and the U.S. are now allies, yet for some the hatred remains. This is the Pearl Harbor excuse.
Despite the fact that in the War of 1812 the British burned Washington D.C., we have forgiven them for that but we haven't forgiven the "Japs" for the Pearl Harbor attack. We keep using the Pearl Harbor excuse for our "Jap-bashing".