Massive economic incentive to expand NATO after end of Soviet Union
All those former Soviet countries were outfitted wit Russian weapons. They had to be refitted with Western weapons (NATO all use the same weapons or something).
To create new markets for the big military equipment companies that wouldn't be buying as many tanks and warplanes etc since Cold War was over (ultimately US didn't buy less because War on Terror started in 2001).
- point raised by Ali Abunimah