The Nazis did not have it easy gaining power. In 1923 they attempted a putsch, called the Munich Putsch, and tried to gain power by force. They thought the political and economic time was right, but the party underestimated, and the coup failed.
Hitler, the leader, learned his lesson, and even when the chance was offered to him later on, he resisted and waited for power to come to him legally.
He bid his time, and that time came ten years later. Germany was in the middle of an economic crises, the state was in a political turmoil, and the people were discontented. Four years earlier, America's stock market had crashed, and the impact of this on Germany was disastrous. American loans were called in as they matured, and new loans ceased. Germany could not afford to live they way it had been, and there was no way to pay off reparations.
The people suffered the most. Inflation shot up - at one stage one American dollar was worth 4 billion German marks. Jobs fell like pins at a bowling alley - there were 6 million Germans unemployed at one time. Slums were common, beggars moreso, and people rummaging in bins were not all that rare either.
Hitler chose these conditions to strike. He campaigned heavily, and eventually he managed to get into the Reichstag. However, there were not enough members of the Nazi party in there with him for him to have good control. He made the rest of the Reichstag feel that it needed a new election, and this time there was an all-out victory. He used his political force to put other parties out of the race, and make people fear him if they did not vote for him.
Once he had the Reichstag under control, Hitler managed to get them to pass the Enabling Act, which made him a legal dictator. In this way he could not be kicked out of the Reichstag for seizing too much power.
Now that he had become dictator, Germany was his. Hindenburg made him Chancellor, formally, and at once Hitler set about doing things his way. Like the persecution of Jews. Like conscription. But he did good things too - completed work on the Autobahns (the German freeways - a national network), and brought unemployment down dramatically. But in the end it was his policies that started World War II. Hitler was probably the worst decision the German people have ever made; but that is not for discussion here.