What is Chauvelin’s plan and why is it so clever and deadly?

In Chapter 24, Chauvelin instructs his henchman, Degas, to return with six soldiers and ambush Percy the moment he walks into the pub. But he says that he wants Percy "alive... if possible." Degas leaves, and Chauvelin waits for him to return.

Chauvelin becomes more threatening once he's on French soil in Calais. When he tells Degas to return with his soldiers, we begin to understand just how much danger Percy is actually in. Indeed, it is not Percy's death at the hands of Chauvelin that we fear, but his simple arrest -- for that ensures his execution at the hands of the masses and guillotine. Therefore Percy's margin for error is much smaller -- he can not win in a hand-to-hand climax with Chauvelin; he cannot succeed simply through brawn and physical prowess. Rather the victory here must be about rescue and physical escape without allowing a face-to-face meeting.