Q: In Martian Chess, one is not allowed to undo an opponent’s move, but what if their move made a capture?

…Moving their piece back to the same spot wouldn’t undo the capture, so is that allowed?

A: To review: the NO UNDOS rule states that one may not “reject” another player’s move. If one player moves a piece across the canal, the other can’t move it back to the same square it came from.

While the rule is called “no undos,” that’s just a shorthand title. It’s the move itself, not the capture, which you are not allowed to reject by sending the piece back to it’s immediately previous location.