Here, I’ll do it for you guys. Assurances were made to USSR that taking down the Berlin Wall wouldn’t lead to more NATO expansion eastward, but again USFP rules the day and there’s been about a dozen nations join since and it’s gotten too close to Russia. This has long been a point of contention for Putin. Paul is right, if we had succeeded in our hastey attempt to admit Georgia and Ukraine, we’d be at war right now with Russia. Russia has legitimate national security concerns in opposing Montenegro’s inclusion into NATO.
Of course the silver lining to a Trump/Putin bromance is that Trump’s not likely to do anything against Putin’s wishes.