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.

Explanation:

relations between the soviet union and the united states were driven by a complex interplay of ideological, political, and economic factors, which led to shifts between cautious cooperation and often bitter superpower rivalry over the years. the distinct differences in the political systems of the two countries often prevented them from reaching a mutual understanding on key policy issues and even, as in the case of the cuban missile crisis, brought them to the brink of war.

the united states government was initially hostile to the soviet leaders for taking russia out of world war i and was opposed to a state ideologically based on communism. although the united states embarked on a famine relief program in the soviet union in the early 1920s and american businessmen established commercial ties there during the period of the new economic policy (1921–29), the two countries did not establish diplomatic relations until 1933. by that time, the totalitarian nature of joseph stalin's regime presented an insurmountable obstacle to friendly relations with the west. although world war ii brought the two countries into alliance, based on the common aim of defeating nazi germany, the soviet union's aggressive, antidemocratic policy toward eastern europe had created tensions even before the war ended.