Transcript for Gen. McChrystal: For a president, visiting troops is not 'time to tout your politics'

I don't know if you folks are aware of the what's happening. We want strong borders in the United States. The Democrats don't want strong borders. Only for one reason, because I want it. Just looking at this warrior group, I think I'll say I don't want the wall. Then they'll give it to me. I figured out the solution. That was president trump in Iraq this week making his first visit to a conflict zone. While the president drew praise for the holiday trip, some say the president went too far in injecting politics. Stanley mcchrystal is one of those critics. He served as the top commander in Afghanistan until 2010 and he's the author of a new book, "Leaders, myth and reality." I sat down with him for his take on the commander and chief. You saw president trump there this week talking to troops, not only in Iraq, but in Germany. The speech clearly took a political turn. He was talking about the border wall. He was disparaging Democrats. What did you think when you saw that? When leaders visit soldiers, young women and men, there's a sacred interaction that occurs. You have to provide for them leadership. You're showing that you are there. You're also listening to their problems. You don't use that as a time to tout your politics or your personal opinions. You use it as a time to reassure them what they're doing is appreciated. It's very important we understand the role and responsibility that leaders have which is sometimes -- sometimes transcends what we want to do in the moment. You saw some of those troops come forward with red make America great hats. The military rules prohibit military personal from showing any political leanings while in uniform. Did they violate that? I think they violated the spirit of it. I think it's unfortunate because if the U.S. Military becomes politicized, it will become something we're not happy with. As we talk today, Syria's military is poised to enter the kurdish held town. What do you think will happen there? Without U.S. Troops. I think there's every likelihood the kurds come up in a very difficult position. We leveraged kurdish military prowess to deal with ISIS and I think we did that well. I think we created an expectation with them which was natural. They thought we would help protect them. I think know that's every likelihood they'll do whatever damage they can to the kurds to remind the kurds they're in an area where they're very vulnerable. Outside of the kurds, what difference does it make if those 2,000 U.S. Forces leave? You never know until we see how things play out. My sense is that we have a uous regime or region now that has a Russian presence which had been out for about 30 years after the 1973 war. Now Russia is back. They're back in an influential way. Iran has increased influence across the region. If you pull American influence out, you're likely to have greater instability. It will be much more difficult for the United States to try to push events in any direction. There's an argument that says we pull up our stuff, go home and let the region run itself. That has not done well for the last 50 or 60 years. I take it you don't believe ISIS is defeated? I don't believe it's defeated. I think ISIS is as much an idea as it is a number of ISIS fighters. There's intelligence saying there's more ISIS fighters around the world now than there were a few years ago. Doesn't mean we didn't do well against ISIS in Iraq and much of Syria. ISIS is an idea. As long as the ground exists, the causes that cause people to flock to a movement like ISIS, you'll have it flare back up again. In Afghanistan the president has ordered them to start looking at drawing down half those troops there. Do you think that's a problem? I think the great mistake in the president's guidance is that just when we were starting to sit down with the Taliban and begin negotiations, he basically traded away the biggest leverage point we have. If you tell the Taliban we're absolutely leaving on date certain, cutting down, leaking ourselves, their incentives to cut a deal drop dramatically. I worry about the confidence of the Afghan people. At the end of the day, that's what determines what happens in Afghanistan. We rocked them in the belief that we're allies that can be counted on. At the same time this is what president trump campaigned on. They have a pretty good idea he wanted to get out of there. I think so. It's not a big surprise. The first thing we have to do is navigate from where we are, not from where we wish we were. Jim Mattis' last day as defense secretary is Monday. You've seen his very public rebuke of president trump. What do you think Americans should make of a resignation of someone like Jim Mattis? I would guess that secretary Mattis took a long time agonizing over writing a letter that was as direct about his feelings as that particular letter was. He knew it would be very public and make a very strong statement that was much broader than the Syria issue. It was about America's role in the world. I personally think it was valuable. Maybe it causes the American people to take pause and say wait a minute. If we have someone as selfless and committed as Jim Mattis resigning 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 why did he do it? We ought to ask what kind of commander and chief he had that Jim Mattis, the good marine, felt he had to walk away? If someone was asked to be secretary of defense right now who you knew, what would you say to that person? I would ask them to look in the mirror and ask them if they can get comfortable enough with president trump's approach to governance and how he conducts himself and would they be able to be loyal to this commander and chief? If there's too much of a disconnect it would be a bad foundation upon which to try to build a successful partnership. If you were asked to join the trump administration, what would you say? I would say no. It's important for me to work for people I think are basically honest, who tell the truth as best they know it. You think he's a liar? I don't think he tells the truth. Is trump immoral in your view? I think he is. What would you say to those Americans, and there are a lot of them, who support Donald Trump, who say I like what he's doing, he's shaking things up, I don't care about this other stuff? I would say everyone has to make their own judgment. I can't tell any supporter of one politician or another that they are wrong. I would ask every American to do is, again, 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 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 background is so shady, if we're willing to do that then that's in conflict with who we are. I think it's necessary at those times to take a stand.

This transcript has been automatically generated and may not be 100% accurate.