There's also Collective Security Treaty Organization (CSTO) that includes Armenia, Belarus, Kazakhstan, Kyrgyzstan, Russia, Tajikistan. It's kind of like NATO, with their own analog of Article 5: attack on one is attack on all, with all the consequences. Belarus and Russia specifically, are also about to sign a new military doctrine as part of the Union State. Whether anyone would fight for Poland... the history seems to suggest that that's a "no."