Apparently it is too late as most of the Members have left The Pact already, but it seems that Ukraine was suggesting that they might join NATO and Russia wasn't having that. Although how this entitles Russia to invade Ukraine is a bit much to take. Russia didn't attack any of the others when they left, but then Russia would have been on a very sticky wicket if they had.
However, Ukraine is right on the border of Russia, along with a couple of other countries who are still in The Pact.
I think Putin has lost his marbles as The West was bound to help to defend Ukraine, but Putin doesn't seem to have seen this coming. And now of course, it has all escalated way beyond proportion.
Funnily enough, I didn't even know where Ukraine was before this happened. So nothing lost as far as I am concerned. The prevailing wind for where I live is Westwards, which means if Putin drops a Nuclear Bomb on Paris all that Nuclear Shite will blow back on Russia. Has Putin thought about this I have to wonder?