> The pilots of both aircraft (civilian and military) are supposed to be keeping a constant visual watch for traffic. The military aircraft should also be keeping an eye on primary radar.

So in your opinion, that was went wrong here, the military/pilot of the refueling plane didn't actually keep visual watch for traffic nor radar?

I wasn't there and don't know all the facts, so I'm not going to attempt to assign blame in this specific instance.

But speaking generally, I'll just say: If you're flying in VMC conditions (good weather), you're always required to see-and-avoid. Even if you're on an IFR clearance. Everything else is just considered an aid for situational awareness.