The command of the South Hungary provincial garrison had requested that some of the Marine battalion stationed in Belgrade, be transferred to the north shore of the Danube to deal with banditry. To comply, fully two companies with supporting weaponry were sent over the afternoon of the 16th and the results were disappointing to say the least. There were two pig thieves captured and one enterprising young man who had brought bushels of plums and late season raspberries from Serbia to sell to fruit-starved bakeries in Novi Sad. The pig thieves were transferred to the authorities but the fruit smuggler somehow escaped. There was no indication what had happened to the seized foodstuffs.

The following morning the unit commanders received a frantic communication from their battalion commander to abandon their police activity and return at once to Belgrade as early morning aviation had identified Serbian forces advancing up the Sava towards Belgrade. Unfortunately it would be some time before the Austro-Hungarian boats could arrive.

From captured records it appears that the fruit merchant and his pig-thieving partners were members of the Black Hand. Their job had been to terrorize the population of the area north of the river to pull some units out of Belgrade. When the fruit merchant "escaped" (with promises to return with subsequent deliveries of fresh fruit from Serbian farms) he quickly radioed the commander of the Belgrade assault force. The resulting information was music to the ears of the Serbian commander and set in motion the final action surrounding Belgrade.

For Trapp this action represented the final part of his activity as commander of the Danube flotilla. His energy and judgment were noticed in Pola and he was quickly reassigned to command of the submarine fleet based in his home town of Zara. The war on the river rapidly subsided as larger land-based forces arrived and the freedom to roam up and downstream became highly limited.

Years afterward Trapp was to write that the war along the Danube had been especially tragic. While the opponents had cultural and historical enmities, the Austro-Hungarian economic miracle had created the possibility of reversing that and creating an atmosphere of tolerance and acceptance. Certainly the upper echelons of the Black Hand would have agreed, although this was, in their case, no benefit. Contemporary accounts indicate that Black Hand rhetoric had moved from portraying the Confederation as a traditional ethnic devil to portraying the Austro-Hungarians as economic imperialists. Given a little more time and a lighter hand than what Pola had desired, conflict might have been avoided altogether along the Danube.

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