As Aldo15 said, if you want a group of soldier to be in a different side instead of the one by-design, you have to give it a leader that is in the side you want fight against.
In your case, first, be sure that resistance has an enemies-side (do you know it can be friendly both west and east?).
So give the group a leader of the enemy side.
If you don't want the leader to be in-game, set its presence to 0%.
There is another way too, but I would suggest you the above one: easier and faster.