The "Budapest Memorandum"[1] article 4 would be invoked: 4. The Russian Federation, the United Kingdom of Great Britain and Northern Ireland and the United Sta

The "Budapest Memorandum"[1] article 4 would be invoked: 4. The Russian Federation, the United Kingdom of Great Britain and Northern Ireland and the United States of America reaffirm their commitment to seek immediate United Nations Security Council action to provide assistance to Ukraine, as a non-nuclear-weapon State party to the Treaty on the Non-Proliferation of Nuclear Weapons, if Ukraine should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used. That doesn't really answer the question, however, since "seek immediate United Nations Security Council action to provide assistance to Ukraine" is not a very strong statement. NATO would not be directly involved (IMHO) because NATO is a mutual defense treaty and (hypothetically) no NATO country would be attacked. However, spillover from an attack into / onto a NATO country would (theoretically) draw all of NATO into the war. [1] https://en.wikipedia.org/wiki/Nuclear_weapons_and_Ukraine#Budapest_Memorandum https://en.wikipedia.org/wiki/Nuclear_weapons_and_Ukraine#Bu...