|I did some double-checking on the nuk/att FCs and what changes and when:|
1. If a planet is set to a regular FC (i.e. aaa or 123) and it is attacked by a ship, the FC will remain the same.
2. If a planet is attacked by Ground Combat, the FC will remain the same. This is so for any FCs (i.e. aaa or 123 or nuk).
3. If a planet has its FC as "nuk/att", the PLANET initializes combat and so the FC is randomized. In this case, even a ship set to "Kill" does not start the battle.
Basically, you have an advantage if YOU initiate the combat. You get to discover what FCs the enemy is using.