@evilgrin
He arranged for the Japanese hunters to back off during the Jeju Island raid, effectively trapping the Korean team inside the ant nest and leaving them to die. If they died, he wanted to use his now superiority in hunter firepower to pressure and potentially "conquer" South Korean.
Unfortunately, Beru-to-be killed Goto Ryuji and most of the Japanese hunters, and our MC saved the day.
The black box contained incriminating evidence (a recording) of the conversation between Goto and the Japanese Association President where they explicitly reveal that they were trying to abandon the Korean hunters.
In the novel, the Japanese president was trying to pressure the Korean president with "We Japanese helped you when you couldn't do it yourself, so you should help us now that we're weak" logic, and the Korean president was having none of that BS.