"America First" would become the driving mantra, and the US would refocus taxpayer dollars on domestic problems rather than foreign ones, he said. Much of the Washington foreign policy establishment worried that Trump would usher in a diminished -- perhaps even isolated -- US posture.
But in his first year as commander in chief, Trump has largely discarded the non-interventionist campaign rhetoric, expanding the US military footprint around the world and demonstrating a willingness to use military force to confront even indirect threats to the US.
"So far, this President has struck with the advocates of strength and action," said Anthony Cordesman, a defense expert at the Center for Strategic and International Studies.
Trump's first year as commander in chief was not a blatant flip-flop, though. Instead, it was the product of competing impulses that have not only defined the military decisions he has confronted in office but also had revealed themselves during the campaign.
While his remarks as a candidate were peppered with notable criticism -- particularly in Republican politics -- of US military engagements in Iraq, Afghanistan and Libya and of proposals to directly confront the Syrian regime, Trump also argued for a more aggressive fight against ISIS.
And on that front, he has followed through, empowering military commanders on the ground to ramp up the fight against ISIS with increased airstrikes and troop levels that have hastened the pace of victory. The moves have led to the ouster of ISIS from its capital in Raqqa
and nearly all of its territory across Syria and Iraq.
Those sweeping victories are now raising new questions about the US's long-term presence in the Middle East. So far, the Trump administration has offered no indication that it plans to pull the more than 7,000 US troops deployed to Iraq and Syria. And beyond ISIS, Trump has greenlighted the expansion of US counterterrorism campaigns in other Middle East countries and in Africa -- even when ISIS is not the target.
US airstrikes against terror groups have more than doubled in Somalia, according to the Foundation for Defense of Democracies' Long War Journal, which tracks US military campaigns.
And in Yemen, the US air campaign largely targeting al-Qaeda in the Arabian Peninsula has delivered more airstrikes than the previous four years combined, according to the Long War Journal.
The swelling US military posture around the world under Trump has served as only the latest piece of confirmation that Trump would not be the non-interventionist president that some had hoped he would be.
The White House did not respond to multiple requests for comment for this story.
Doug Bandow, a senior fellow at the Cato Institute who's a prominent proponent of a non-interventionist US military policy, said he had "some hope" as he listened to Trump lament US foreign military entanglements during the campaign.
"To the extent you could rely on rhetoric, he looked a lot better than his opponent," Bandow said. "He was making some of the right critiques."
But whatever hopes Bandow had have been crushed repeatedly in the last year with Trump's decision to strike a Syrian airbase with 59 Tomahawk missiles, followed by the President's Afghanistan decision, which Bandow called "a signature moment that suggested whatever his gut feelings were going to be, we weren't likely to see them resolved in policy."
While there was always "an inherent contradiction" in Trump's rhetoric lamenting US foreign military engagements while calling for a stepped-up military campaign against ISIS, "at least there was a contradiction," Bandow said.
Even in December 2016, a month after his election, Trump continued to promise an end to a policy of "intervention and chaos."
"We don't want to have a depleted military because we're all over the place fighting in areas that we shouldn't be fighting in. It's not going to be depleted any longer," he declared.
The contradiction hasn't evaporated altogether, though, as exemplified by the debate over the US strategy in Afghanistan, which stretched months longer than initially anticipated due to Trump's misgivings about deepening US involvement in the war and the debate over its merits that roiled his top advisers.
It's a contradiction that Trump acknowledged directly
as he announced his decision to sign off on a strategy that would send several thousand more US troops to fight in a conflict entering its 17th year.
"My original instinct was to pull out -- and, historically, I like following my instincts," Trump said. "But all my life, I've heard that decisions are much different when you sit behind the desk in the Oval Office; in other words, when you're President of the United States."
What has also changed is the coterie of experienced foreign policy advisers and military generals that now surround the President, namely his national security adviser, Lt. Gen. H.R. McMaster, his defense secretary, James Mattis, who's a retired four-star general, and his chief of staff, retired four-star Gen. John Kelly.
"This President has appointed a national security adviser, a secretary of defense and a secretary of state who all are very pragmatic but who also really believe in asserting American strength and acting more decisively than the previous administration," said Cordesman, the Arleigh Burke Chair in Strategy at the Center for Strategic and International Studies.
But while the results of his first year signal that Trump has sided with his instinct for a muscular US military posture over that of limiting US military exposure abroad, the half-dozen foreign policy experts CNN spoke to for this story said the administration's long-term national security strategy remains uncertain.
That has largely been a product of the contradictions in Trump's rhetoric, but also is due to his penchant for making impulsive and unclear, if not contradictory, statements about his administration's policy.
"I think people are right to be worried that the President's thinking is either contradictory or unsound and this is one of the costs of the President's undisciplined behavior," said Kori Schake, a fellow at the Hoover Institution who co-authored a book with Mattis and was openly critical of Trump during the campaign. "I think it is a mistake to believe this is going to smooth out and the President is going to have a clear, consistent policy."