Let me paint a scenario in your head to get a better idea of what I mean:
A clump of my marines are getting shot from the cliff above by a garrisoned sniper. The sniper has the vertical advantage so my marines can’t engage. I send my own sniper around to engage from behind. My sniper won’t attack the garrison. It will just sit there while the garrisoned unit keeps sniping my marines.
The only way my sniper will engage is if:
A) My marines engage the garrisoned unit
or
B) Garrisoned sniper engages my sniper