
Di penjara, Hitler menulis memoarnya, Mein Kampf (Perjuanganku). Setelah bebas tahun 1924, Hitler mendapat dukungan rakyat dengan mengecam Perjanjian Versailles dan menjunjung Pan-Jermanisme, antisemitisme, dan anti-komunisme melalui pidatonya yang karismatik dan propaganda Nazi. Setelah ditunjuk sebagai kanselir pada tahun 1933, ia mengubah Republik Weimar menjadi Reich Ketiga, sebuah kediktatoran satu partai yang didasarkan pada ideologi Nazisme yang totalitarian dan otokratik. Tujuan Hitler adalah mendirikan Orde Baru hegemoni Jerman Nazi yang absolut di daratan Eropa. Sampai saat itu, kebijakan luar dan dalam negerinya bertujuan mencapai Lebensraum ("ruang hidup") bagi kaum Jermanik. Ia memerintahkan Jerman dipersenjatai kembali dan Wehrmacht menginvasi Polandia pada bulan September 1939, menyebabkan pecahnya Perang Dunia II di Eropa.
Di bawah pemerintahan Hitler, pada tahun 1941 pasukan Jerman dan sekutu Eropanya menduduki sebagian besar Eropa dan Afrika Utara. Tahun 1943, Jerman terpaksa bertahan diri dan mengalami serangkaian kekalahan dalam pertempuran. Pada hari-hari terakhir perang, saat Pertempuran Berlin berlangsung tahun 1945, Hitler menikahi kekasih lamanya, Eva Braun. Tanggal 30 April 1945, kurang dari dua hari kemudian, keduanya bunuh diri agar tidak ditangkap Angkatan Darat Merah, lalu mayat mereka dibakar. Kebijakan Hitler yang supremasis dan termotivasi oleh ras mengakibatkan kematian sekitar 50 juta orang selama Perang Dunia II, termasuk 6 juta kaum Yahudi dan 5 juta etnis "non-Arya" yang pemusnahan sistematisnya diperintahkan oleh Hitler dan rekan-rekan terdekatnya.

Hitler's Nazi Party became the largest elected party in the German Reichstag, leading to his appointment as chancellor in 1933. Following fresh elections won by his coalition, the Reichstag passed the Enabling Act, which began the process of transforming the Weimar Republic into the Third Reich, a single-party dictatorship based on the totalitarian and autocratic ideology of National Socialism. Hitler aimed to eliminate Jews from Germany and establish a New Order to counter what he saw as the injustice of the post-World War I international order dominated by Britain and France. His first six years in power resulted in rapid economic recovery from the Great Depression, the denunciation of restrictions imposed on Germany after World War I, and the annexation of territories that were home to millions of ethnic Germans, actions which gave him significant popular support.
Hitler actively sought Lebensraum ("living space") for the German people. His aggressive foreign policy is considered to be the primary cause of the outbreak of World War II in Europe. He directed large-scale rearmament and on 1 September 1939 invaded Poland, resulting in British and French declarations of war on Germany. In June 1941, Hitler ordered an invasion of the Soviet Union. By the end of 1941 German forces and their European allies occupied most of Europe and North Africa. Failure to defeat the Soviets and the entry of the United States into the war forced Germany onto the defensive and it suffered a series of escalating defeats. In the final days of the war, during the Battle of Berlin in 1945, Hitler married his long-time lover, Eva Braun. On 30 April 1945, less than two days later, the two committed suicide to avoid capture by the Red Army, and their corpses were burned. Under Hitler's leadership and racially motivated ideology, the regime was responsible for the genocide of at least 5.5 million Jews, and millions of other victims whom he and his followers deemed racially inferior.