There are several treaties that cover relations between NATO nations, Russia, and the former USSR nations of eastern Europe. Minsk treaty, and Budapest treaty, are two that come to mind, because Putin has mentioned bot of those as being violated. But, there are more.

To be brief- NATO agreed to stay out of eastern Europe after the breakup of the Soviet Union, and to not militarize the former Soviet republics.

Look at eastern Europe today, and you can see that our western leaders are disgusting, and reckless fucking liars, with zlllions of dollars of western tax payer money funding the money-grubbing military industrial complex all throughout the former Soviet Union. And, in clear violation of treaty agreements.