Defense Secretary Jim Mattis' resignation Thursday — and President Donald Trump’s retaliatory swifter removal Sunday — raised concerns as to whether anyone could now serve as a check on the president.

“I was at the airport yesterday,” said former Secretary of Defense Chuck Hagel, who served under President Barack Obama. “I had five different people recognize me, came up to me, all private citizens all doing different things in their lives, not attached to government or politics at all. They are all very much afraid and asking the question … is there nothing to stop this guy?”

“That feeling is pretty strong across the country and is legitimate,” Hagel said.

From an institutional perspective, Congress is that check through its responsibilities for oversight and budget. The withdrawal of troops from Syria, however, which was reportedly the basis for Mattis' departure, and the drawdown from Afghanistan, which is popular among Trump’s base and among many troops and veterans who have questioned the need to continue the now 17-year war — Trump will not need congressional approval to execute both maneuvers, said Anthony Cordesman, who holds the Arleigh A. Burke Chair in Strategy at the Center for Strategic and International Studies.

"As commander-in-chief he certainly enough discretionary authority within the current airlift program,” Cordesman said. “He certainly can do that.”

In other matters, such as ending the U.S. commitment of troops to Korea, Congress would have to weigh in, Hagel said.

“In strategic policy issues, the president does have constitutional powers in these areas – but they don’t go unchecked,” Hagel said. “Congress is really the answer as much as anything. Congress is going to have to come out of its shadow and will really have to step up to their constitutional responsibility of oversight with Trump on these big, broad national issues.”

But for service personnel, what if they receive an order they think may be illegal?

For military officers, first they can try to steer the commander in chief to a legal alternative. The head of U.S. Strategic Command, Air Force Gen. John Hyten, was asked just that last year. "If it’s illegal, guess what’s going to happen? I’m going to say, ‘Mr. President, that’s illegal,” Hyten told the Halifax International Security Forum in 2017. “'Guess what he’s going to do? He’s going to say, ‘What would be legal?’ And we’ll come up with options, of a mix of capabilities to respond to whatever the situation is, and that’s the way it works. It’s not that complicated.”

However, Trump has previously viewed alternative proposals as evidence that his top generals were not respecting his decisions or authority, such as his 2017 direction to assassinate Syrian President Bashar Assad. The incident was first reported in Bob Woodward’s book “Fear,” and independently confirmed to Military Times by a former U.S. official who witnessed the incident.

In that case, Mattis simply refused to push the order forward.

“That story is true,” the official said. “I viewed it as the essence of loyalty, Mattis making sure he [Trump] was not going to do something illegal, and he did it in a very loyal way.”

“[Trump] wanted to assassinate a foreign leader, which you can’t do,” the official said.

Under the Uniform Code of Military Justice, officers and enlisted are to follow the president’s or a superior’s orders, unless that general order “is contrary to the Constitution, the laws of the United States, or … or for some other reason is beyond the authority of the official issuing it.”

Both Hagel and Cordesman emphasized that they thought it was unrealistic that Trump would make any drastic decision, such as ordering the military to execute an unprovoked missile launch.

“I don’t think we are anywhere near that point now,” Hagel said.

However, if military personnel “really find you have an order you believe is illegal, you can disobey,” Cordesman said. “It will immediately bring some aspect of a courts-martial.”

Cordesman said that in a courts-martial, between the provisions in the UCMJ and public opinion, it would be unlikely an officer or enlisted would be found guilty. Either way, it would be costly to the nation, he said.

“A courts-martial with any significant number of troops would become a political nightmare for everyone,” Cordesman said.

The alternative — if military personnel follow what is an unlawful order because the president ordered it — will come back on them, Hyten said.

“You will go to jail,” Hyten told the Halifax conference. “You could go to jail for the rest of your life.”