Pearl Harbor was an attack on the US by Japan that precipitated the US entry into WWII. It occurred on Dec 7, 1941. World War II was a conflict that militarily involved many nations of Europe, Asia, and North America that most historians agree began with the German attack of Poland in 1939. Japan was engaged in Empire-expanding conflicts with China and other Asian and Pacific nations at the same time. The first allies were Poland, Great Britain and Northern Ireland, and France. These forces allied to declare war on Germany following the invasion of Poland.