Countries fought, loads of people died, and the good guys won.
Basically, the Germans were given a line in the sand and told if they crossed it, there would be war. By invading Poland, they crossed it, so France, and the UK (amongst others) declared war on Germany. Germany had Italy, and Japan amongst it's allies, and the Allies had a whole host of countries. After Japan attacked the USA, they joined in on the Allied side. Germany had also attacked the Soviet Union, so, they came in on the side of the Allies as well.
All this began in 1939, and by 1945 the Germans were defeated, and later that year, the Japanese as well. Rather than seeking war reparations as had happened in previous wars, the Allies actually helped Germany, and Japan get back on their feet. Germany was split into East, and West after the war, with the West under the influence of the UK, USA, and France, and the East under the Soviet Union.
Near the end of the war, it was discovered that Germany had been systematically eliminating large segments of their populations, including around 6 million Jews, and 5 million other "undesirables," including Gypsies (Romani), homosexuals, disabled people, Slavs, and many others. This became known as the Holocaust.