Term | Explanation |
| MAC | Mediterranen Air Command. |
| Magic | Code name for US code breaking operations against the Japanese. |
| Maginot Line | Series of sophisticated French fortifications positioned along France's eastern borders consisting of steel and concrete anti-tank emplacements and pillboxes. |
| MAN | German armed anti-nazi resistance group, named after the MAN engineering works in Bavaria. |
| Manhattan | Cover name (actually Manhattan Engineer District) for the development of the atomic bomb in the USA. |
| Maquis | Name given to groups of the French Resistance (after a type of foliage in Corsica, behind which resistance fighters on the island used to hide). |
| Marita | Operational code name for the German attack on Greece in April 1941. |
| Market-Garden | Operational code name for the Allied operation intended to establish a bridgehead across the river Rhine at the town of Arnhem in the Netherlands. |
| Menace | Operational code name for Allied operations at Dakar - September 1940. |
| Merkur | Mercury - Operational code name for the German air attacks on Crete - April 1941. |
| Message Personnels | System used to communicate with members of the Resistance on the European mainland from Britain by using coded messages broadcast on BBC radio. |
| MI5 | British security service responsible for national security matters within the United Kingdom. |
| MI6 | British security service, otherwise known as the Secret Intelligence Service, responsible for collecting foreign intelligence relating to national security. |
| MI9 | British service responsible for assisting escaped prisoners of war. |
| Milice | Paramilitary police force of Vichy France (Milice Francaise). |
| Millennium | Operational code name for the RAF bombing of Cologne - May 1942. |
| Mincemeat | Operational code name for British deception operation in April 1943 where false operational plans were planted on a corpse and left floating off the coast of Spain. |
| Moonshine | British device used on aircraft to fool German radar into seeing far more aircraft than actually existed. |
| MTO | Mediterranean Theatre of Operations. |
| Mulberries | Transportable floating harbours constucted from hollow concrete, towed to the D-Day landing sites and used to offload equipment and supplies during the Normandy landings. |