I don't know what the reality is, but we had a ground army in the 1990's, that could have done exactly what geezer mentioned above, parking troops on the border as a trigger, if we'd had the courage, if we valued long term stability. But after the end of gulf 1, I believe the perception is that we don't have a ground army anymore, at least not in the traditional sense. We have occupation forces, light infantry units, special forces, driving around in armored cars. Or a shorter version - in 1990, we were a 1st world force ready to take on another 1st world force. Now, we're built around trying to suppress 3rd world nations, like a bad version of the UK trying to keep colonies, and we're not ready for a 2nd world force.
Not to be too political, but during the entire cold war, even the worst leaders were capable of going head to head with the soviets. They had to. The stakes from 1945-1990 were our existence. Over the past 30 years, we haven't had to worry about that, and we've lost the ability to play at those stakes. Technically, we could have put the 82nd Airborne in, just on friendly exercises, last week. With maybe a phone call reminder that it would take all of about an hour and a half for the Russian navy to cease to exist is we find ourselves in a shooting war. But we don't have the political capability to do anything like that anymore.
So now we don't lead the world, we don't make the decisions, we just watch. That's too bad, because the world doesn't need another USSR. And I agree, at this point it's too late to do anything about the Ukraine.