Because the other side has nuclear weapons and declaring war brings those into play as well. Which is why we've been in a cold war with the Soviet block instead

Because the other side has nuclear weapons and declaring war brings those into play as well. Which is why we've been in a cold war with the Soviet block instead of a real war. The prospect of nuclear war was mutually unacceptable. So, they both sides fought lots of proxy wars instead. And those didn't stop when the wall fell. The Ukraine is a proxy war that has Russia backed by allies China, Iran, and others opposing Nato backed Ukrainian forces. Most of the conflicts in the middle east involve most of the countries I've mentioned and lots of their allies as well. Syria is the forgotten conflict where Iran and Russia are partnering and where the US, Israel, and Europe have failed to make progress for more than a decade. With Ukrainian forces now striking deep in Russia, that cold war is heating up. This is a reminder that that can work both ways and might soon start affecting Nato territory as well. All deniable of course. But a few high profile attacks like this sends a not so subtle message that Russia can escalate as well if they want.