the germans also did quite a lot of tank development in violation of versailles, building a series of 'grosstraktor'- literally large tractor- pretending they were for agricultural use. they just happened to have spots for armor and guns on them, too.
they then sent these to the soviet union who were happy to let them refine their armor doctrine and train with tanks away from prying eyes in germany in exchange for technical assistance in their own armor programs. no word on whether the soviets came to regret this arrangement.