However, the forts themselves were only part of the Dutch defence system; political and diplomatic treaties were far more important, particularly since Britain could not allow a hostile power to control ports in Northern Flanders like Ostend. Britain's agreement to act as guarantor of the treaty and to provide the Dutch with military support against any aggressor ultimately proved to be far more effective than the Barrier itself. Nevertheless, during the French Revolutionary Wars, French forces invaded and overran the Dutch Republic and established the Batavian Republic. The Netherlands remained directly or indirectly under French control for nearly two decades.