The economic crash of the USA holds a lot of blame, considering it helped Hitler gain popularity. If it hadn't been for the crash, Hitler's Nazis would've been a minor and insignificant political group. The Russian Revolution was idly involved, for the wave of savage communism that had taken place in it scared the middle and higher classes in Germany. If Germany became communist, it would be their end. Hitler promised he would abolish communism (even though the Nazis stand for Nationalsozialismus, in other words socialism, which is much like communism).
The failure to appease Germany was also a main cause. Appeasement means 'pacify: cause to be more favorably inclined; gain the good will of;'. It was actually the opposite. The League of Nations (especially Britain) appeased Germany too much. They let it break the Treaty of Versailles, which was in essence good, for it was causing a lot of problems to the German society, but it also let Hitler do whatever the heck he wanted. During the 1930s, many politicians in both Britain and France came to see that the terms of the Treaty of Versailles had placed restrictions on Germany that were unfair. Hitler's actions were seen as understandable and justifiable. When Germany began re-arming in 1934, many politicians felt that Germany had a right to re-arm in order to protect herself. It was also argued that a stronger Germany would prevent the spread of Communism to the west. In 1936, Hitler argued that because France had signed a new treaty with Russia, Germany was under threat from both countries and it was essential to German security that troops were stationed in the Rhineland. France was not strong enough to fight Germany without British help and Britain was not prepared to go to war at this point. Furthermore, many believed that since the Rhineland was a part of Germany it was reasonable that German troops should be stationed there. In May 1937, Neville Chamberlain became Prime Minister of Britain. He believed that the Treaty of Versailles had treated Germany badly and that there were a number of issues associated with the Treaty that needed to be put right. He felt that giving in to Hitler's demands would prevent another war. This policy, adopted by Chamberlain's government became known as the policy of Appeasement. The most notable example of appeasement was the Munich Agreement of September 1938. The Munich Agreement, signed by the leaders of Germany, Britain, France and Italy, agreed that the Sudetenland would be returned to Germany and that no further territorial claims would be made by Germany. The Czech government was not invited to the conference and protested about the loss of the Sudetenland. They felt that they had been betrayed by both Britain and France with whom alliances had been made. However, the Munich Agreement was generally viewed as a triumph and an excellent example of securing peace through negotiation rather than war. This famous picture shows Chamberlain returning from Munich with the paper signed by Hitler declaring 'Peace in our time.' When Hitler invaded the rest of Czechoslovakia in March 1939, he broke the terms of the Munich Agreement. Although it was realized that the policy of appeasement had failed, Chamberlain was still not prepared to take the country to war over "..a quarrel in a far-away country between people of whom we know nothing." Instead, he made a guarantee to come to Poland's aid if Hitler invaded Poland.
The failure of the League of Nations was also a mayor reason. Although the idea for the League of Nations had come from Woodrow Wilson, there was a change of government in the United States before the signing of the treaty and the new Republican government refused to join. As a punishment for having started World War One, Germany was not allowed to join and Russia was also excluded due to a growing fear of Communism. Other countries decided not to join and some joined it, but later left. The main weapon of the League when Hitler began gaining power was to ask member countries to stop trading with an aggressive country. However, this did not work because countries could still trade with non-member countries. When the world was hit by depression in the late 1920s countries were reluctant to lose trading partners to other non-member countries. The League had no army either, a reason why it didn't do anything against the Japanese invasion in China. Soldiers were to be supplied by member countries. However, countries were reluctant to get involved and risk provoking an aggressive country into taking direct action against them and failed to provide troops. The League was also comprised of many members who couldn't agree on one thing. The Council of the League of Nations only met four times a year and decisions had to be agreed by all nations. When countries called for the League to intervene, the League had to set up an emergency meeting, hold discussions and gain the agreement of all members. This process meant that the League could not act quickly to stop an act of aggression.
Hitler was merely an advocate of all this tumult and he took advantage of the harsh situation Europe was in. Hitler could not have done what he did without a country in turmoil. He was simply a man in the right place at the right time, and would never have made it into history books if Germany's condition had been better.