Hitler rearming Germany
The Treaty of Versailles signed at the end of WWI included a clause defending Germany from rearming. This clause was violated and Germany undertook a covert plan to develop weapons again.
When Hitler was appointed Chancelor in 1933, he accelareted the process and on 16 March 1935, he announced publicly the restoration of a military draft and a plan to create an army of half a million soldiers...
This was no longer covert operation and while most European governments condemned Hitler's undertaking, their disaproval was not followed by any actions nor sanctions...