Erfworld 66: page 60

[teratorn]

Parson wants to win the war, by winning the battle, not just finishing Ansom. When survival was at stake, he would go for it, but now he'll want to crush this army. He can truly defeat Ansom.

Unless Parson is a total idiot at handling the Tool (and he isn't, as evidenced by his willingness to let him take credit for the trap), he's planning to hit Ansom (though perhaps he also plans to hit some or all of the siege as a bonus).

I see it the other way around.
Parson "See how he's afraid and he thinks these dwagons are going after him?"
Stanley "Yeah let's do it."
Parson "You're smarter than that."
Stanley "Am I?"
Parson "Yes, that is what he expects, but you'll do something else."

EDIT: the thing is, Parson gets Ansom but loses most of his dwagons and lets siege within the column. The alliance could keep going to the city then.