Martial law, not "marshal" law.

"Marshal" is an ancient loanword from Old (Norman) French, cf. modern French maréchal, which in turn is borrowed from Old Frankish *marhskalk "stable boy, keeper, servant," still evident in Middle Dutch maerscalc, marscal "id.", modern Dutch maarschalk "military commander" (the meaning influenced by the French). It is cognate with Old High German mar(ah)-scalc "id.", modern German Marschall "military commander" (the meaning influenced by the French).[3] It originally meant "stable keeper," from Germanic *marha- "horse" (cf. Engl. mare) and skalk- "servant" (cf. Old Engl. scealc "servant, soldier").[4] This "stable servant" origin is retained in the current French name for farrier: maréchal-ferrant.
The late Roman and Byzantine title of comes stabuli ("count of the stable") was adopted as a Latin analogue, which has become English "constable" (cf. French (obsolete) connétable).
(Wikipedia - marshal)

"Martial" comes from "Mars" and refers to the military or army.