I like the idea of minefields in the game, but laying them should be a province upgrade, kind of like naval fortifications. An accuracy note would have to be that mined paths can damage your ships just as well as your enemies. Incorporate a minesweeper unit as a counter to the development to maintain balance. If you really want to give offensive capabilities to the mine weapon then that should be a secondary option for your destroyers / submarines who were primarily responsible for mine deployment during WWII.
Logic is the path to the Dark Side Mr. Spock.