Adolf Hitler had a strong hatred for Jews, he suffered a lot in his life, and whenever he suffered there was always a Jew responsible. He grew very anti-Semite.
Anyways, years later Hitler now being a member of the Nazi Party, he was made fuher of Germany. Which means he was now in charge of Germany, could whatever he wanted for as long as he wanted. His hate for the Jews started the Holocaust, the Holocaust started in 1933. If you've ever read Anne Frank, she was raised in Germany, but escaped to the Netherlands when she was a few years old to avoid going into concentration camps.
Adolf Hitler had a plan to build an empire and kill all possible Jews in it.
How WWII started.
Hitler wanted to invade Poland. Britain said that if he was to invade Poland, they would declare war. In 1939, Hitler ignored the threats and invaded. WWII has now started.
Hitler also invaded France and won the battle in 1940. He attempted to invade the UK and Russia (then known as Soviet Union) but failed.
In 1941, Japan bombed Pearl Harbour, attacking American ships and killing American soldiers. America then joined the war.
In 1944, plans made by British Prime Minister Sir Winston Churchill, invaded France which was occupied by Nazi Germany. The British, American and remaining French army that escaped to Britain took part in the battle. This battle was called D-Day. If you've seen Saving Private Ryan, the battle scene on the beach was meant to be D-Day. WWII veterans described the battle to be very accurate to the real battle.
Anyhow, the Germans lost D-Day, and the Brits, French and USA walked into France. They went through many fights in France as they attempted to walk into Germany and stop Hitler.
Meanwhile the Soviet Union was taking out the Polish side of Nazi Germany, they walked in and took over all the concentration camps owned by Germany, freeing the Jewish slaves. On the other side, the British and Americans were doing the same. The Russians were the first to walk into Berlin where Hitler shot himself, not wanting to give in to the Russians, his body was taken outside and burned while Russians were literally shooting only 500m away at other German soldiers.
Hitler shooting himself, the Nazi's then surrendered.
Meanwhile, America was at the same time also fighting with Japan, they dropped two atomic bombs on Hiroshima and Nagasaki, threatening to drop another one on Tokyo the capital city. The Japanese surrender, the war ended on September 2nd 1945.