WARNING: The following contains spoilers for Episode 39 of That Time I Got Reincarnated as a Slime, "Ramiris's Warning," now streaming on Crunchyroll.
Reincarnated as a Slime's Rimuru Tempest knows that to survive an isekai world, you need to make powerful allies. In Season 2, he must also learn to handle international politics, and the return of Veldora the storm dragon is a potential complication on the world stage -- as well as Rimuru's newfound status as a True Demon Lord.
Tempest's major victory against the Falmuth/Church invasion army came at the cost of many Falmuth lives, and now Rimuru must explain this event to the world's other nations. He cannot reveal the truth about himself and Veldora, so in Reincarnated as a Slime Episode 39, he develops a cover story to set up Veldora as a deterrent against further aggression.
In Episode 39, the leaders' meeting continues even after Ramiris arrives to warn of impending disaster, and Rimuru gets everyone's attention to announce his new plan. He and king Gazel Dwargo know they should not casually reveal the real origins of Rimuru's True Demon Lord status to the world, nor can they share the true circumstances of Veldora's return. The reality is that Rimuru slaughtered 20,000 Falmuth and church troops to fuel his ascent to True Demon Lord status, but he plans to explain all those deaths as simple casualties in a conventional battle against Tempest's defenders.
Other national leaders, he says, will not be troubled by news of a bloody but otherwise ordinary battle. Rimuru further explains he will blame Veldora's return on all the carnage taking place in that fight. Then, according to the cover story, he and other leaders paid a steep cost to placate Veldora and convince him to become an ally of Tempest.
Rimuru's plan accounts for why so many Falmuth and Church troops vanished, and if their story is accepted among other national leaders, the Tempest nation will successfully appear innocent in this affair. According to them, the Church and Falmuth kingdom unfairly attacked them, thus Tempest is innocent and not a sincere threat. This plan also vaguely explains Rimuru's ascent to True Demon Lord status.
Most importantly, Reincarnated as a Slime Episode 39 shows that Rimuru plans to use Veldora's presence and strength as a deterrent against aggression from other nations. Veldora is a feared and legendary name and few, if any, other nations would attack Tempest for fear of retaliation. Despite having an ordinary human form, Veldora still has the power necessary to back up this threat, and Rimuru expects the Falmuth kingdom will be the last to ever brazenly attack Tempest. This buys him and his allies time to gather their forces and face their many enemies elsewhere, including Demon Lord Clayman and Hinata Sakaguchi of the Western Holy Church.