On December 7th, 1941 the United States of America was attacked by Japan in Pearl Harbor, Hawaii. This would be America's entry into World War 2 - a war that America had managed to avoid becoming entangled in for over 2 years by this point.
To give you more perspective, by December 1941 the Nazis had already invaded and occupied a significant portion of France. This was 6 months after the Nazis invaded the Soviet Union on June 22nd, 1941 in "Operation Barbarossa".
On December 8, 1941, President Roosevelt declares war on Japan, the day after Japan bombed Pearl Harbor. Roosevelt gives a speech to a joint session of congress.
2 hours into the attack, a KGU reporter makes the only known live report of the attack by telephone to NBC in New York City.