From the game manual:
Minefield –
Whenever a non-Engineer unit enters a Minefield hex, a mine attack is immediately carried out against it. Minefields come in three strengths: 1, 2, and 3, which represent the strengths corresponding to the “odds” line of the Combat Results Table that the attack is resolved on; i.e., a “3” Minefield attack is executed on the 3:1 line. Mines can be removed during play by a non-Disrupted Engineer unit that begins its turn in the Minefield hex. Mined hexes provide no defensive benefit to any unit occupying such a hex.
HSL