An opponent that I was playing had the Warriors of Ulric deployed in his quest zone, and when his combat phase came around he began combat and declared a zone of attack against me in order to move his Warriors into his battlefield.

At the time he declared the zone that he was attacking, he had no units in his battlefield who could attack me, but we could not find in the rules where that is a requirement to start combat, so we played on.

You can.

1- You declare the target of the attack.

Then

2. Declare Attackers

The active player decides which of his units in his
battlefield (if any) are going to participate in the at-
tack.

(cf. core rules).