During the final scene of Hunter Killer, Gerard Butler's character gets a Russian warship to fire two missiles upon Russian defense headquarters, blowing up the bad guys and ending the saga and movie.
How did he do this?
NOTE: OBSA (One Big Spoiler Alert)
His character relied on common sense of all seamen, as he saw it. There are several instances throughout the movie where his character makes statements to his crew as to why he's doing things the way he is, which, while unpopular at the outset, made sense and was bought into after the explanation. He used this same common sense seamanship to bring the conclusion which happened.
With the ending, he took a gamble, relying on his instincts as a veteran of the seas. He didn't know the Russian ship was going to fire on the base, but was hoping for some kind of resolution which didn't require his boat to fire. While he was given the order to fire from Chairman of the Joint Chiefs of Staff Donnegan, he knew it would create bigger issues and further hostilities. Having the Russian Captain Andropov onboard and believing in him was a good step towards making this happen. Without Andropov, it probably wouldn't have turned out the same way. He trusted the Captain to help him and hoped the Captain trusted him in the same way. Andropov's intervention with the crew of the Russian vessel allowed him the best course of action. It wasn't that he forced someone else to fire, he only hoped it would happen without his having to do it. In the end, everything worked out for the best (ex machina?).