"Marches to cannons mouth" is a phrase that refers to a dangerous and risky situation that someone willingly walks into. There are a few synonymous idioms that express a similar sentiment. For example, "dancing with death" suggests that someone is taking serious risks that could lead to their demise, while "playing with fire" implies that they are doing something that could lead to serious consequences. Another close synonym is "jumping into the lion's den" which conveys the idea of putting oneself in a perilous situation with potential deadly consequences. All of these phrases show the seriousness of a dangerous situation that someone has willfully entered.