Think about those decaying old counter-strike systems that are keyed to launch when American nukes are detected in-flight, from the Cold War era. Even our idiotic Dept of War knows better than to test those by firing nukes in Russia’s general direction, because there’s a very good chance we’d get bombarded and we already blew our missile defense stockpile protecting greater Israel.

presumably they'd use a submarine-launched nuke given the circumstances

Pakistan too, since they would not be receptive to nuclear fallout and millions of refugees.