Putin said that NATO's enlargement towards Russia "would be taken in Russia as a direct threat to the security of our country".

Security Dilemma in action: One party wants to strengthen their own security, the other party considers that a threat to theirs and responds in kind.

Instead of mutually agreeing that they've both reached a point of military capacity where actual war would be more costly than lucrative, the respective leaders conveniently overlook who would be paying that cost and keep posturing, and the arms dealers keep making bank.