https://www.matrixgames.com/forums/tm.asp?m=4996958
The rule says:
Port Blockade – Small islands may be blockaded by a single naval groups. All other hexes can be blockaded by 3 naval groups. To blockade a port the fleet must be 1 hex away and able to move into that port from that hex if they controlled it and have no enemy fleets next to that port as well under the same conditions.
@Alvaro, I just would like to confirm two things:
1. Does a friendly naval group into the port being blockade prevents the blockade by the enemy fleet?
2. Does a friendly air group into the port being blockade prevents the blockade by the enemy fleet?
Thanks