by creating the ansom in a hole, he forces ansom to defend himself at the cost of the column, and because he thought he was going to croak the dwagons he didnt bother to organise some basic anti dwagon measures (siege surrounded by every marbit crossbowman in sight and then some)

the siege is wide open

ansom must protect himself or risk being swarmed

When Ansom defends himself but is revealed to be not the target and that command has got the stanley plan wrong YET AGAIN making 3 in a row,
it will create psychological cracks in the alliance as the less dedicated elements start to think why are we fighting with this looser whos having rings run round him by a moron and whatever sycophant he's dredged up from somewhere,
result the alliance that Ansom himself described as unnatural and tennuous fractures and breaks removing far more units for far less cost possibly only the 3 dwagons already croaked,
And the Siege gets dry roasted,

Its not just about the physical war,
its about the psychological war as well