Retired Army Lt. Gen. Mark Hertling is criticizing Donald TrumpDonald TrumpHow Florida explains our polarized politics Dem senator: Pardoning targets of Russia probe would be 'crossing a fundamental line' Trump lawyers looking into special counsel's potential conflicts of interest: reports MORE for the Republican presidential nominee's comments about giving the U.S. military 30 days to come up with a plan to defeat the Islamic State in Iraq and Syria.

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"I heard the very simplistic orders that he gave in terms of giving 30 days to find a plan to soundly and decisively defeat ISIS," Hertling, who served in the Army for 37 years, said Tuesday on CNN.

"I had to ask myself, 'What the hell does he think we've been trying to do for the last 14 years in terms of al Qaeda?'"

During a rally in North Carolina, Trump said he would order the military to come up with a plan to defeat ISIS "in 30 days" if he becomes president.

“I am going to convene my top generals and give them a simple instruction: They will have 30 days to submit to the Oval Office a plan for defeating ISIS,” Trump said.

Hertling said Trump's order shows a "complete lack of understanding of the threat and the ways to fight it."

"It's a sophomoric approach to elements of national security policy, because if he's just calling in the military, he's missing the point that there are several other elements of national security that will help defeat ISIS," he added.

"There have been some very good advances made against this element of terrorism, and he might want to take a look at some other countries and the way they've fought terrorism."

Hertling said the fight against ISIS will go on for a long time, calling it a "generational threat."

"You just don't say, 'Beat it in 30 days,' because it's frankly insulting," he said.