Sen. Ruben Gallego (D-Ariz.) fiercely called out the president for not “understanding how the military works” after Donald Trump suggested that the military use some of the country’s “dangerous cities as training grounds.”

“Look, the president’s an idiot,” Gallego told CNN’s Boris Sanchez Tuesday before claiming that Trump “doesn’t actually understand how the military works.”

“The first thing it’s going to be is that most U.S. citizens will stand against that in civil protests, and you’ll have many of us joining them in that if they try to do such a thing,” he continued.

Gallego, a Marine veteran, went on to argue to Sanchez that troops dispatched to these cities “will not be firing upon their own men and women, their own neighbors” due to the “oath that our members of the military take.”

“Only, again, an idiot like Donald Trump would believe that such a thing could happen,” he added.