Why did Hitler have to rearm in secrecy?

Nazi Germany conducted a cautious policy of rearm in the 1930s because the provisions of the Treaty of Versailles prohibited the development and possession of advanced systems of weapons. Germany was to have a small army - some 100,000 men -, and could not have tanks, airplanes and other destructive weapons. Adolf Hitler won the collaboration of the Soviet Union to allow German designers and troops to use Soviet fields to prove the new tanks, for example, in the years before WWII.

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