Because Russia also has borders, and violating that changes the game from a defensive war to an offensive one. The international calculus changes in a way that

Because Russia also has borders, and violating that changes the game from a defensive war to an offensive one. The international calculus changes in a way that makes it less of a principled stance — opposing Russia because it does this stuff is a pretty defensible position. It does lead to shipping a few divisions of “advisors” to Ukraine, but that’s not here nor there.