Tell me, how the fu*ck can anyone guarantee a competely different country (Russia) that some other country will never be member of a certain alliance?
You raise an important question. The existence of these miniature states (yet bound in all the vestiges of sovereignty) is a pernicious problem in European history for the past few centuries.
Ordinarily, the answer to your question would be economic coercion and brute force. Unfortunately this path is now only available to the United States. Even China struggles to address regional problems in this way.
The only real answer to the problem is that the many countries of the world (200 is it now?), should be consolidated back into a manageable 30-45 countries. Everyone is responsible for their own little patch of dirt, and doesn't expect ludicrous assurances from each other - as Tacitus wisely pointed out.
A Polish Intermarium could be one of those 30-45 countries. With Poland's abundant natural resources, well developed stock market, impressive demographics, and powerful military - it could easily be the locus of a Central European superpower.