From a 1994 interview with Henry Kissinger and former Ambassador Jack Matlock:
Kissinger: And if we are reluctant to antagonize the Russians today when they're weak, it is an illusion to think that somewhere three, four years down the road, we can suddenly move in that direction when they're stronger. Nor am I saying that we should treat the Russians as enemies. What I'm saying is we should have the friendliest political relations with Russia. I look at NATO as a safety net, as an insurance policy, and you don't want to wait to take out an insurance policy when the house is already on fire.
Matlock: There is one other factor here that we seem to be forgetting, and we did, though it was not a legally binding assurance, we gave categorical assurances to Gorbachev, back when the Soviet Union existed, that if a United Germany was able to stay in NATO, NATO would not be moved eastward. And, you know, I think that the current Russian government is very clear. It is not legally binding, but it was, you might say, a geopolitical deal. And if we simply ignore it, then I, certainly if I were a Russian, it would be hard for me to interpret this, even though it may not be intended that way, and it is not, as anything less than an attempt to shut Russia off from Eastern Europe.