There was, however, some activity on the river itself. Belgrade was now garrisoned by a mixed force of Serbians, British, and French, the British being mostly gunners, who had been detached, together with some big naval guns, from the British navy. For some time before they arrived the Austrian monitors and picket boats had again been patrolling the Danube and annoying the Serbians, but the Belgrade garrison put an end to the activities of these vessels with their big guns. The British sailors especially rendered good service by means of a small picket boat commanded by Lieutenant Commander Kerr. Though armed with only a single machine gun, this small boat was so persistently troublesome to the enemy that it earned for itself the name "Terror of the Danube." Of dark nights it would poke its way into creeks and passages, alarming the Austrians constantly and causing them no little loss. Once it even succeeded in persuading one of the monitors to pursue it into a carefully prepared mine field, over against the Serbian shore, with the result that the monitor was permanently put out of action. But these operations were of minor importance just then. For now Serbia was called on to face a new enemy, in some of its aspects much more terrible than the Austrians, for it demanded a sort of fighting in which the Serbians were not so well trained. The Austrians had, indeed, left behind them an ally that was to accomplish as much mischief almost as they themselves had caused the Serbians.