The Free City was vaguely under League of Nations control and had no native military force - it was just barely a 'free city' and was a bit more like Hong Kong in China today. While the city was overwhelmingly German in character Polish authorities were primarily in control.

For the most part the status of 'co-combatant' can probably be chocked up to two things - the incorporation into West Prussia and the cooperation of local SS sympathizers and agents. Local forces and police were partially responsible for removing Polish authorities, with the 15-hour siege of the Polish-controlled post office being of note. The Polish in the office - mostly civilians armed with light sidearms - were tried and executed.

Sadly, much of the Polish minority in the city was decimated by the Germans, with Poles being deemed inhuman and many being subjected to cruel and unusual experiments, such as sterilization and castration.