before invading Poland (which started the second world war), Hitler invaded and occupied czechoslovakia, he also remilitarized the rhinelands and started rebuilding the german army
Famously, the British PM at the time, Neville Chamberlain desperately wanted to stop a new world war and had a policy that people called 'Appeasement
you can read much much more about it in this thread by /u/true_new_troll
edit:
talks about the rearmament of the german armies by the weimar republic and hitler by /u/kieslowskifan