Budapest Memorandum calls for talking (consulting) if a border violation occurs
...the legal obligation on the part of the United States and Russia and the UK is, under paragraph 1, “to respect the independence and sovereignty and the existing borders of Ukraine.” But what happens if one state does not do this? The only specific obligation is under paragraph 6, which provides that the parties “will consult in the event a situation arises that raises a question concerning these commitments.” So, clearly there is no obligation to use force. The requirement under paragraph 5– to seek action by the Security Council– seem to be triggered only if Ukraine is a victim of aggression or the threat of aggression when nuclear weapons are involved. That does not seem to be the case here, and even if it were, the obligation would be to take the matter to the Security Council, where, of course, Russia has a veto.