Retired Army Col. Peter Mansoor, who served as David Petraeus's right hand in Iraq, says Donald TrumpDonald TrumpMcMaster to South Korea: US will pay for missile defense system Comedian Hasan Minhaj blasts Trump, media at WHCA dinner White House correspondents' chief: 'We are not fake news' MORE would be a "foreign policy disaster for the United States."
"He'd be a loose cannon who would shoot from the hip. He would endanger America's status in the world, he would dismay our allies and empower our adversaries," Mansoor said in an interview with The Hill. "Other than that it'd be a great idea to have him as president."
Mansoor served 26 years in the Army, including as executive officer to Petraeus during the Iraq surge.
Trump has prompted criticism from retired members of the military for advocating for torture beyond waterboarding and the killing of terrorists' families. Torture is against international law.
"I think waterboarding's torture. I would have refused to have done it," said Mansoor, who is currently the General Raymond E. Mason, Jr. Chair in Military History at Ohio State University.
He said he opposed torture as a commander because he didn't want his troops subjected to the same measures.
He also pushed back against Trump's argument that the Islamic State in Iraq and Syria (ISIS) has done far worse to its prisoners, noting that enemies throughout history also employed brutal practices.
For example, he said, the Japanese burned U.S. troops alive and conducted live organ removals without anesthesia during World War II.
"In some cases the Germans tortured prisoners, and the Japanese often tortured prisoners, a third of our prisoners died in Japanese captivity," he continued.
"But we retained the moral high ground and that enabled us to build a post-war order based on liberal democratic values... based on the rule of law. And based on values enshrined in Western civilization and in our Constitution.
"If we were to all of a sudden become no better than the people we seek to kill, then we lose any kind of claim to be, you know, the standard bearer for the global order," Mansoor added.
"The key is in the past we've had leaders who didn't stoop to their level, they retained the moral high ground which enabled the United States to craft a post-war order to our benefit."
Mansoor has also signed another open letter by foreign policy experts opposing Trump's candidacy.
Former CIA and NSA Director Michael Hayden, a retired four-star general, paved the way for criticism against Trump, after telling TV host Bill Maher that U.S. troops could refuse to carry out his orders.
Mansoor said there was some grey area, however, on whether actions would violate international law based on the orders.
"If you say, 'Well, we say we're going to kill this terrorist's family. You know, someone might disagree with that. But if the order is, "Well, you're going to hit this target because there's a munitions cache in it' -- some of them will agree to that, still knowing that the real purpose is to kill the terrorist's family," he said.
"So, yeah, there's some grey areas, but if there is a clear order to disobey the laws of war, someone, hopefully someone in the U.S. military will disobey it and then it will become public knowledge, and then there will be a shit storm the magnitude of which we can't comprehend in the international community," he added.