Comment by aguaviva

It's not like a line in the sand, admitted as such by both sides, was broken, one with explicit promises that it wouldn't be.

Indeed it's not, because that's an extremely distorted and misleading narrative. For example, on multiple occasions (notably 1994 and 1997) Russia signed treaties validating NATO expansion long after this supposed "explicit promise" (which also wasn't quite what you seem to think). We also have statements from the two most important players on the Soviet side (Gorbachev and Shevardnadze) thoroughly discounting this version of events.

Whatever source you got that narrative from is simply misinformed, or worse.