I suspect Parson is going to attack and "defeat" Ansom (be that kill or capture, as required by the rules of Erfworld) and ignore the column. He can leave the main army alone because, if he can take out Ansom, he doesn't need to worry about it.

Ansom is the coalition leader. He's not just a warlord, but the one who forged the Alliance (as seen in the case of Charlie). Parson doesn't need the Alliance to auto-lose with the loss of its leader, he only needs for it to dissolve.

The clue is here: http://www.giantitp.com/comics/erf0044.html

Specifically:

Stacks without a leader are forced to autoattack when in contact with units from non-allied capitals.

The story play-through of this system effect might be as follows.

With news of Ansom's loss, the coalition first hesitates, then dissolves. The units in the column will either panic, or blame each other for the predicament. Skirmishes will break out, and those that don't initiate out-right hostilities will withdraw in such an disordly stampede that they effectively (from a system point of view) "attack" each other by dealing significant damage to each other.

This would in no way mean the end of the strip either. We're only on Part 1. In the aftermath of the Alliance's destruction, the world might descend into chaos and total warfare, providing the dramatic crucible for the next story arc.