During this portion of the war the front, especially in a secondary region such as the Balkans, was highly fluid. Forces from both sides frequently crossed into enemy territory to disrupt the communications of the enemy or to interdict the movement of forces. The Serbs took advantage of Trapp's actions in the Iron Gates to try to establish a blocking force on the Danube at Smerderevo, crossing some Marines and shore guns and having the opportunity to fortify the guns.

Obviously, the Serbian movement had to be countered. Both sides used control over the riverbank as a way to jam their opponent into a smaller section of the river and providing themselves with the freedom to deploy forces to either side of the river. Trapp had used mines and the capture of the western edge of the Iron Gates for that exact purpose. The surprise was that the Serbians had the audacity to return the favor.

In the documents recently located in Novi Sad, Trapp had noted to his aide that he had permitted himself to be tempted by the chance to destroy the Allied fleet within the Gates. Such an expedition was strategically meaningless. When the predictable complaint arrived from Pola with the admonition that the Serbian lodgment must be removed at once the only response was a terse selbstverständlich.

Counters and maps from River Fleets may be used to play Blue Danube scenarios.