I tried to declare a protectorate over the Hanoi region (which contains Tonkin units). The Tonkin revolted and started an ongoing struggle that I can't finish.
I can't try for the protectorate again because there's a hostile Tonkin unit in Hanoi. There's also a ton of French units (mine) in the region. The Tonkin unit has no cohesion and fights only for a second (taking a few hundred casualties out of its total of over 12,000 men). It can't seem to retreat to another region and it doesn't seem to be suffering any casualties from lack of supply or lack of cohesion. It just sits in Hanoi, practically invincible, turn after turn.
What can I do about this unit?
Is there a way to delete it (perhaps playing as Tonkin one turn and disbanding it or maybe editing the savegame file to delete it)?
This almost seems a bug. It certainly is a great way to prevent most colonial decisions from being used in Hanoi.