What did Hitler do wrong in the war?

If you mean "What did Hitler do that was morally wrong?" the answer is that he single-handedly started World War II and as a result he was solely responsible for the deaths of 60 million people including the 6 million Jews that he ordered to be killed.

If you mean "What mistakes did Hitler make?" here is the answer:

Hitler jumped the gun by invading Poland in 1939. Hitler wanted a right-of-way across the Polish Corridor so that he could build a highway and a railroad to access the eastern part of Germany. The Polish Corridor was German territory that had been given to Poland after World War I, so that the Poles could have access to the German port of Danzig which was made into an international port under the rule of the Poles. The Corridor divided Germany into two parts and the Germans could only access the eastern part by boat since the Poles refused to allow the Germans to cross the Corridor.

Hitler wanted Danzig returned to Germany because it was a city that was almost 100% German and the German inhabitants did not want to live under the rule of the Poles. Hitler had previously negotiated with the British to give the Sudetenland to Germany because there were 3.5 million Germans living there.

World War I ended when the Germans signed an Armistice based on American President Woodrow Wilson's 14 points. Point number 5 was that each ethnic group in Europe would be allowed to live in whatever country they chose. The Sudeten Germans wanted to live in Germany and Neville Chamberlain agreed to allow the Sudetenland to be given to Germany.

The Poles were refusing to negotiate for the return of Danzig to the Germans and to allow a right-of-way across the Polish Corridor. Hitler was becoming frustrated and he decided to invade Poland and just take the Corridor and Danzig. This is where he made a big mistake because if he had just waited, he would have eventually gotten what he wanted. He had the law on his side because Germany had signed an Armistice based on Wilson's 14 points, including Point number 5 which could have been invoked to settle the Danzig and the Corridor dispute.

When the British realized that negotiations between the Germans and the Poles had broken down, they made a guarantee that they would support Poland in case of attack. Then the Germans made a pact with the Soviet Union and the British changed their guarantee to include their support only in case of a German attack.

The British realized that the Russians were going to join the Germans in an attack on Poland and they did not want war with the Soviet Union, only with Germany. At this point, Hitler should have realized that the British wanted war and they were going to use the German dispute with the Poles to declare war on Germany. Hitler wanted to be allies with the British and the French and with America. He should have backed off at this point and not invaded Poland. Since the start of the negotiations, the Poles had killed 58,000 ethnic Germans in Poland, but so what? This was no reason to invade Poland.

By becoming allies with the Soviet Union for the purpose of invading Poland, Hitler had to make concessions to the Soviets which resulted in giving a big chunk of Eastern Europe to the Soviets for their "sphere of influence," which meant allowing the Soviets to dominate Eastern Europe, including one third of Poland.

Hitler's second big mistake was in invading the Soviet Union in June 1941. The invasion of the Soviet Union meant that Germany now had to fight on two fronts. America was allies with the Soviet Union and had been helping the Soviets for years. Hitler should have known that he couldn't beat the Soviet Union and America, while still fighting the British in the West.

Before invading Russia, Hitler had tried to make peace with the British and had failed. He should have waited longer and eventually the British would have accepted peace negotiations. Hitler should have continued his efforts to become allies with America.

When Hitler invaded the Soviet Union, the Germans took back the eastern one third of Poland where most of the Jews in Europe lived. Half of the Jews who were killed in the Holocaust were from the countries that the Germans "liberated" from Communism when they invaded the Soviet Union. These countries included Estonia, Latvia, Lithuania and Belarus. Hitler made a big mistake in letting the Soviet Union take over these countries in the first place. If he had not made a pact with the Soviet Union in order to invade Poland, the Jews in Eastern Europe would probably not have been part of the Holocaust.