Donald Trump returned to office as president on Monday, reassuming his duties as commander in chief. His position once again puts him in charge of the world’s most powerful military, which — often at his discretion — can either engage in lethal warfare or act as a deterrent and a force for peace.

How Trump behaved as commander in chief during his first administration may offer a guide for the next four years.

In Monday’s inauguration speech, Trump set the tone for a significant change in U.S. foreign policy that could have seismic implications for America’s friends and foes.

“Our armed forces will be free to focus on their sole mission — defeating America’s enemies,” he said.

His recent comments about taking over the Panama Canal and Greenland — refusing to rule out the use of military force to do so — raised eyebrows, along with referring to neighboring Canada as “the 51st state.” He also signed an executive order to rename the Gulf of Mexico as “Gulf of America” on Monday.

“It was an intent to grab that media attention and put himself at the center of the news cycle,” retired Lt. Gen. Douglas Lute, a former U.S. ambassador to NATO, told ABC News. “This unpredictability means that all paths lead back to the Oval Office and his next statement.”

However, Trump’s slogans about making America great and putting America first may soon bump up against his need for the world to see him as a dealmaker. The war in Ukraine has been raging since 2022, but he’s promised to bring it to an end.

“Shortly after I win the presidency, I will have the horrible war between Russia and Ukraine settled,” he said last August. “I’ll get it settled very fast.”

Trump warned Russian President Vladimir Putin that if the war doesn’t end soon, Ukraine could get substantially more aid.

At the same time, he has told Ukraine it won’t receive more U.S. support unless it enters into peace talks. Last week, White House budget director nominee Russell Vought declined during his Senate confirmation hearing to fully commit to doling out $3.8 billion in aid that Congress has already approved.

China’s threats against Taiwan and international shipping may also embolden Trump to show Chinese President Xi Jinping that America believes it has the upper hand militarily.

“He will always defer to a solution that makes him appear strong and in charge,” Lute told ABC News. “So I don’t think that there’s an easy way to simply walk away from a big competition with China.”

The situation at the border between the U.S. and Mexico is also top of mind, with Trump declaring a national emergency in his inaugural address.

“I will end the practice of catch and release and I will send troops to the southern border to repel the disastrous invasion of our country,” he said.

The biggest difference in American foreign policy may be seen in the Middle East, and the war between Israel and Hamas. On Jan. 7, he created a deadline for a Gaza ceasefire deal in exchange for Hamas releasing hostages they took in the Oct. 7, 2023, attack on Israel.

“If the deal isn’t done before I take office, which is now going to be in two weeks … all hell will break out,” he said during a news conference.

Then, last week, the outgoing Biden administration — along with incoming Trump envoys — helped broker a ceasefire and hostage exchange that went into effect on Sunday. However, the agreement is fragile and fraught with pitfalls. In particular, it remains unclear who will govern Gaza if the peace lasts.

The images on the streets betray an uncomfortable reality. It’s still the gunmen of Hamas in charge — a situation unacceptable to the Israeli government. The Trump administration is also unlikely to accept Hamas being in control.

Beyond Gaza, the situation in the Middle East has gone through considerable changes since Trump was last president. Iran, a major power in the region, has seen many of its allies weakened by various conflicts.

Hamas has been left devastated and weakened by months of conflict with Israel. Lebanese political party and armed group Hezbollah — designated as a terror group by the U.S. — is a shadow of its former strength, with its leader Hassan Nasrallah killed in an Israeli strike in September.

Former Syrian President Bashar Assad also fled from his country, his family held power for more than 50 years, after his regime was toppled by a rebel offensive in December.

Amid this new balance of power, Trump wants to normalize diplomatic relations between Israel and Saudi Arabia. Such a pact could bring peace and stability in the region — the Biden administration was working to broker this agreement prior to Hamas’ 2023 attack on Israel.

The Trump administration faces immense international challenges — and America going it alone may not be a viable option.