Symbolism, Imagery, Allegory
Guess what happens to you when you ride into Mexico with forty men and try to conquer the whole bloody country? Yep, you end up with your severed head inside a jar for all to see. It's fair to say that the severed head of Captain White stands as a warning to anyone who might want to follow his example. And it seems to work, as one Mexican prisoner says:
That's the worst thing I ever seen in my life. (5.166)
Another prisoner doesn't take the same approach to White's head. Instead, he says:
Somebody ought to of pickled it a long time ago. By rights they ought to pickle mine. For ever takin up with such a fool. (5.167)
In other words, this guy has no sympathy for Captain White or his grim fate. He knows that Captain White was a fool to go on such an insane mission and he (the prisoner) was a fool for going with him.