NATO

http://www.theatlantic.com/international/archive/2014/03/no-american-weakness-didnt-encourage-putin-to-invade-ukraine/284168/ : Baker promised Soviet leader Mikhail Gorbachev that if the Soviets allowed Germany to reunify, NATO—the U.S.-led Western military alliance that took form after World War II—would not expand “one inch” further east, not even into the former East Germany itself.

https://www.spiegel.de/international/germany/bonn-moscow-ties-newly-released-documents-shed-fresh-light-on-nato-s-eastward-expansion-a-5a362292-dfe6-4355-b90f-10d635d7d664