The alliance offered at the direct time of the mission was stopping the spread of Red Lyrium sailing around the globe via their Dreadnaught, which is basically a Frigate or Armada probably carrying hundreds of soldiers. This is detailed within conversation at what had been supplied objectively and why inquistion support was required for this mission.
If the Dreadnaught sinks, by allowing it too. More ships will continue to sail that cargo, despite of stopping a single vessel within this mssion. This is detailed in conversation. Finally upon later returning the inquistion close the port
Also later offered upon completion of the objective. Through gaining alliegance. The Ben-Hesserth supply agents in stopping certain Vinatori activity in the regions. This includes assassination attempts on corrupting Vinatori officals throughout the regions.
Their aid at this present time, was directed to the threat of both Red Lyrium from the Red Templars, and Vinatori, pawns of Coryphaeus. Somewhat explained when giving you aid in this current objective.
The alliance on offer here was helping to defeat certain common goals at the direct time of this mission.
I am pretty sure prior to this mission when chatting with Iron Bull or Krem after recruiting them there was a previous Ben-Hesserth operation within Ferelden.