Or was the question about the legitimate rationale for NATO as opposed to the empirical regularity?
AHA ... if it was about the rationale, that would be to anchor a cordon sanitaire around the Soviet Union. That rationale was more than a little thread-bare by the 70's, of course, and with the collapse of the Soviet Union, no longer exists. However, if NATO is rolled up, then the ratio of comfortable to sucky postings in the US global base network will tilt even further toward sucky, and further there will be a serious risk that a majority of US foreign policy will slip from the hands of the Pentagon back to the State Department. So NATO is also there to prevent those things happening. I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.