World War II 1939-1941Timeline By tim Grace 1939 Hitler invades Czechoslovakia Sep 1, 1939 Hitler invades Poland Sep 3, 1939 Britain and France declare war on Germany. 1940 Hitler invades Denmark and Norway 1940 Blitzkrieg Lightning Warfare by Hitler 1940 Italy enters war on Axis side 1941 Italy and Germany attack Yugoslavia Dec 7, 1941 Pearl Harbor Japan attacks Pearl Harbor, U.S. base in Hawaii. Dec 8, 1941 Britain and U.S. declare war on Japan