If he refuses, then we will make him surrender." "Why ask him, when we know he'll refuse ?" inquired Pon.
"Why, we must be polite, whatever we do," explained the Scarecrow.

"It would be very rude to conquer a King without proper notice." They found it difficult to write a message without paper, pen and ink, none of which was at hand; so it was decided to send Pon as a messenger, with instructions to ask the King, politely but firmly, to surrender.
Pon was not anxious to be the messenger.

Indeed, he hinted that it might prove a dangerous mission.

But the Scarecrow was now the acknowledged head of the Army of Conquest, and he would listen to no refusal.

So off Pon started for the King's castle, and the others accompanied him as far as his hut, where they had decided to await the gardener's boy's return.
I think it was because Pon had known the Scarecrow such a short time that he lacked confidence in the straw man's wisdom.