In 1939 after years of tension Germany invaded Poland in 5 weeks the west side of Poland was conquered and on Sept 16 the Soviet Union invaded Poland and Germany conquered Warsaw and the remaining Polish troops left the country to Romania on Oct 5 Poland was finally conquered.
After the invasion Britain and France declared war on Germany but their response was very weak on try it the pacifist way which fail miserly and only strength Germany.
Meanwhile Japan had just invaded China in 1937 they had beef with each other after Japan annexed Korea in 1910 and China got there butts beaten by Japan.
In June 1941 even though at the begging of the war they were allies Germany suddenly invaded the Soviet Union with 3 million troops the largest invasion in history and for the first few months it look like Soviet Union was going to get conqoured.
In December 1941 Japan reaction to the United States banning oil imports Japan bombed Pearl Harbor and destroyed 1/3 of the U.S navy in response the United States declared war on Japan.
In 1943 the German forces arrive at Stalingrad there mission was to cut off the oil rich southern Russia they had to go through Stalingrad and this was the deadliest battle in the whole war and end in German defeat