The former top commander of U.S. and international forces in Afghanistan, retired four-star Army Gen. Stanley McChrystal, criticized President Donald Trump's behavior and handling of the presidency, saying the commander-in-chief is dishonest and immoral.

“I don’t think he tells the truth,” McChrystal said in an exclusive interview on “This Week” when asked by Co-Anchor Martha Raddatz if he believes the president is a liar.

“Is Trump immoral, in your view?” Raddatz asked.

“I think he is,” he said.

McChrystal said he couldn't tell any of Trump's supporters "that they are wrong," but added, "What I would ask every American to do is ... stand in front of that mirror and say, 'What are we about? Am I really willing to throw away or ignore some of the things that people do that are -- are pretty unacceptable normally just because they accomplish certain other things that we might like?'

"If we want to be governed by someone we wouldn't do a business deal with because their -- their background is so shady, if we're willing to do that, then that's in conflict with who I think we are. And so I think it's necessary at those times to take a stand."

McChrystal, a 34-year veteran of the U.S. Army, served as the head of the Joint Special Operations Command from 2003 to 2008 and later assumed command of all international forces in Afghanistan in June 2009. President Barack Obama relieved McChrystal of that command in June 2010 after Rolling Stone published controversial comments McChrystal made in an interview about the administration. In doing so, Obama still praised the general's "deep intelligence" and "love of the country."

McChrystal, who recently published a book on leadership styles throughout history called, “Leaders: Myth and Reality,” criticized Trump for not embodying effective leadership.

“The military talks about would they come for you. And what that means is if you're put into a difficult military situation, would that leader sacrifice himself, put himself and others at risk to come for you? I have to believe that the people I'm working for would do that, whether we disagree on a lot of other things. I'm not convinced from the behavior that I've seen that that's the case here,” said McChrystal.

He also cautioned anyone who might fill the vacancy left by Defense Secretary James Mattis’ departure, to consider if their values sufficiently align with those of the president.

"I think maybe it causes the American people to take pause and say, wait a minute, if we have someone who is as selfless and as committed as Jim Mattis resign his position, walking away from all the responsibility he feels for every service member in our forces and he does so in a public way like that, we ought to stop and say, 'OK, why did he do it?,'" McChrystal said on "This Week."

“I would ask [potential candidates] to look in the mirror and ask them if they can get comfortable enough with President Trump's approach to governance, how he conducts himself with his values and with his worldview to be truly loyal to him as a commander in chief and going forward,” McChrystal said. “If there's too much of a disconnect then I would tell him I think it’s -- it would be a bad foundation upon which to try to build a successful partnership at that job.”

McChrystal said he would not take a job in the Trump administration if he were asked.

"I think it's important for me to work for people who I think are basically honest, who tell the truth as best they know it," he said. "I'm very tolerant of people who make mistakes because I make so many of them -- and I've been around leaders who've made mistakes ... but through all of them, I almost never saw people trying to get it wrong. And I almost never saw people who were openly disingenuous on things."

He also disagreed with Trump’s approach to his visit to Iraq last week to address troops, saying the president was wrong to politicize a usually non-political occasion. In addition to talking extensively about domestic political issues in his speech to troops, Trump autographed troops' “Make America Great Again” hats. The president said in a tweet that the hats were not provided by the White House.

McChrystal said he understood why many young troops would want signed memorabilia from the president, comparing it to meeting a celebrity, but also warned that it “violated the spirit” of the military code and that the military’s apolitical status should be preserved.

“If we encourage young military members to be Republicans or Democrats or anything particular, you start to create schisms in an infantry platoon,” McChrystal told Raddatz on "This Week."

“I never knew who was a Democrat or Republican and even when we were generals, when you got in a room, you never talked about politics because it was just considered bad form," he said. "I think if we allow it or encourage it, I think we are going to create something that could be a slippery slope.”

McChrystal also disagreed with Trump's announcement to withdraw U.S. troops from Syria, warning it would lead to "greater instability" in the Middle East.

"What difference does it make -- does it really make, if those 2,000 U.S. forces leave?" Raddatz asked.

“If you pull American influence out, you're likely to have greater instability and of course it'll be much more difficult for the United States to try to push events in any direction. There is an argument that says we just pull up our stuff, go home, let the region run itself. That has not done well for the last 50 or 60 years,” McChrystal said.

In announcing the withdrawal from Syria earlier this month, Trump touted victory over the Islamic State, or ISIS, there, declaring, “We have defeated ISIS in Syria.”

McChrystal disagreed, citing the continued threat of ISIS’ ideology.

“I don't believe ISIS is defeated,” McChrystal said. “I think ISIS is as much an idea as it is a number of ISIS fighters. There's a lot of intelligence that says there are actually more ISIS fighters around the world now than there were a couple of years ago.”

ABC News has also reported that Trump plans to reduce U.S. forces in Afghanistan by half, about 7,000 troops. In a statement to Bloomberg Friday, Garrett Marquis, a spokesperson for the National Security Council said Trump "has not made a determination to" to withdraw troops from Afghanistan or "directed the Department of Defense to begin the process of withdrawing" troops. Marquis did not respond when ABC News requested further comment.

"Do you see that as a problem?" Raddatz asked McChrystal.

“I think the great mistake in the president's leaked guidance is that just when we were starting to sit down with the Taliban, just we were starting to begin negotiations, he basically traded away the biggest leverage point we have. If you tell the Taliban that we are absolutely leaving on a date ... their incentives to try to cut a deal dropped dramatically,” McChrystal said.

McChrystal added that the decision could have a lasting impact on the trust in the alliance between the United States and the democratically-elected Afghan government it supports.

“Of course I was worried about the confidence of the Afghan people because at the end of the day, that's what determines who wins in Afghanistan,” McChrystal said. “And I think we probably rocked them -- we rocked them in their belief that we are allies that can be counted on.”