"I will end your civilization" is not credible. He'd lose a war powers vote and likely be removed if he even started down that path. To say nothing for the logistical impossibility.

He's not doing some Scott Adams master persuader nonsense. He spent a month being ignored by his counterparty so he just kept amping up the rhetoric until he was threating actual genocide. With human shields placed around the infrastructure he promised to attack, the president desperately begged Pakistan to broker a ceasefire with two sets of terms.