There are several synonyms that can be used to describe someone who "marches to the cannon's mouth," or someone who is brave and unafraid in the face of danger. Some alternative phrases include "fearless warrior," "valiant hero," "courageous soldier," and "brave defender." Each of these synonyms conveys a sense of strength, bravery and determination in the midst of fear or danger. Regardless of which synonym is used, it is clear that the individual being described is someone who is willing to put themselves in harm's way to defend what they believe in or protect those they care about.