"Each Military Unit must complete its move and Operation before Activating the next Military Unit".
George has correctly identified a key concept. Only one Military Unit is in action at a time so a single defending unit is all that is required to destroy it.
All that being said, I'm a bit fuzzy on what happens if a third player is added to the mix on the defender's side. In what order do the defenders decide to make a stand? Clockwise from First Player? Is the owner of the hex given some special consideration?
There is no defined rule about this. I do think that it is a little bit of a corner case but a good think to work out anyway. Here are some thoughts.
- If I were one of the defenders I would prefer to have the other players kill each other off and leave my unit.
- I think this may come up when two allied players have pieces that are defending together.
- Defenders should have the opportunity to discuss the choice.
- I agree that the owner could have special consideration. I would say that it would be an advantage to have the owner have the last chance to defend the tile.