Appearing on MSNBC, Trump biographer David Cay Johnston — who is also an economist — laughed at the president’s proposal to raise steel and aluminum tariffs and reminded viewers that during his real estate developing days Trump always sought out cheap foreign steel.

Speaking with AM Joy host Joy Reid, Johnston said Trump had no idea how his abrupt announcement will affect world’s economy and went on to say that Trump really knows nothing about finance despite “being given” a degree in economics.

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Host Reid noted, “Many people fear a prolonged trade war that could hamper the economy. What they’re doing right now is preparing for a trade war as Donald Trump is tweeting ‘When a country, USA, is losing million of dollars on every country it does business with, trade wars are good and easy to win.’ Is that true?”

“I think Donald Trump is channeling General George Custer,” Johnston scoffed. “Let’s go to war with no preparation and no idea of what we’re facing, and we all know how well that turned out for Custer’s troops.”

With Reid turning back to the Trump’s steel tariffs, she asked, “We just put up the top suppliers of steel to the United States and, as you said, Canada, Brazil, South Korea and Mexico are one, two, three, four. For aluminum it’s Canada, Russia, United Emirates and China. Did Donald Trump use American steel, by the way, when he was building his buildings?”

A laughing Johnston replied, “He went to great lengths, as [journalist] Kurt Eichenwald sussed out to use Chinese steel.”

“This is classic Donald, ‘Do as I say and not as I do,” the biographer quipped, “And let’s keep in mind, Donald was given a degree in economics — I use that verb deliberately because he was given a degree in economics by Penn. And this and a lot of other things demonstrate he doesn’t have any idea what he’s talking about.”

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After breaking down a history of tariffs that led to disastrous results, Johnston circled back to mocking Trump’s intelligence.

“Donald doesn’t know anything, and I mean that literally,” he remarked. “He is completely ignorant about things like this and his ignorance is just appalling and it’s really damaging to us.”

Watch the video below via MSNBC: