The title of this blog post says it all. It was December 7, 1941, at 7:48 a.m. that the Empire of Japan attacked the United States of America at Pearl Harbor, Hawaii. 2,403 Americans were killed while 1,178 others were wounded.
There are numerous good online resources about the attack. An intriguing one that I found recently was a radio news report made shortly after the attack by a reporter who witnessed it. You can listen to it here.