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President Donald Trump pumps his fist after addressing the crowd during his swearing-in ceremony on January 20, 2017 at the US Capitol in Washington, DC. Mandel Ngan / AFP - Getty Images

Trump, who upended politics with his inflammatory nationalist message and takes office as the most disliked incoming president in memory, could have picked any or all of these paths. He went with Reagan's approach, but substituted his own brand of belligerent nationalism for the late president's principled conservatism.

Where Reagan smoothed the rougher edges of his vision with soaring patriotic imagery, Trump walked down a dark dystopia in which crime, joblessness and foreign exploitation had laid the country low while a shadowy elite profited at every turn.

"This American carnage stops right here and stops right now," he said.

Beyond a perfunctory nod to his predecessors in the White House, Trump did little to reassure Americans opposed to his candidacy. And far from lowering the sky-high expectations he set on the trail, he raised them in bold terms that could haunt him if he fails to meet his own standards. At one point, he promised to "eradicate completely from the face of the earth" the threat of "radical Islamic terrorism."

Trump's political ascension was essentially a hostile takeover of the GOP, whose leaders overwhelmingly opposed his candidacy, sometimes in apocalyptic terms. Even after he secured the nomination, a number of senators along with former presidents George W. Bush and George H.W. Bush refused to endorse him. And even after winning the presidency, few Republican elected officials are willing to embrace his policy vision without qualification.

Trump celebrated these differences in his address, indicating he would chart an independent course from the GOP mainstream embodied by Vice President Mike Pence and hold fast to the same message that excited his fans at rally after rally.

The gulf between Trump and the normal order was especially vast on foreign policy. Where modern presidents of both parties used their inauguration addresses to emphasize cooperation abroad in defense of human rights, Trump promised retrenchment and protection from "the ravages of other countries, making our products, stealing our companies, and destroying our jobs."

John F. Kennedy spoke of a "grand and global alliance, North and South, East and West, that can assure a more fruitful life for all mankind" in his inaugural address. George W. Bush declared, "America remains engaged in the world, by history and by choice, shaping a balance of power that favors freedom."

Trump, by contrast, hailed the "right of all nations to put their own interests first" and assured foreign citizens that America would merely "shine as an example" rather than "impose our way of life" on others.

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Trump's speech came after a particularly divisive election. He said he would prosecute and jail his opponent Hillary Clinton, who attended the ceremony, then backed away from his threat after the election. He spent years spreading a false conspiracy theory alleging that President Obama was born in Kenya and then renounced it with little explanation late last year. He proposed banning all Muslims from entering the United States, baselessly accused an Indiana-born federal judge of bias due to his "Mexican heritage," and threatened to sue a group of women who accused him of unwanted sexual advances.

Trump spoke of "solidarity" on Friday and said Americans would "rediscover our loyalty to each other," but there was no significant olive branch to the majority of voters who supported his opponent or to the skeptics within his party who support him reluctantly.

Instead, he promised a unity based on concrete achievements, in which "a new national pride will stir our souls, lift our spirits and heal our divisions" once America achieves economic prosperity. He offered only hints as to what policies would achieve these goals.

Health care, a potentially defining area in which he recently promised "insurance for everybody," did not merit a mention. Trump talked up major investments in infrastructure, a goal that has drawn more support from Democrats than Republicans in recent years.

Trump has never held office before, meaning there's never been a way to hold him accountable for his actions and words outside the court of public opinion. But soon, his words will represent the office of the presidency and his actions will move policy. For some, they may mean the difference between life and death. The results he asked Americans to judge him by on Friday wealth, personal safety and national security will be carefully measured by his opponents, who will be eager to hold him to account.

Far from shrinking from this responsibility, Trump welcomed it.

"The time for empty talk is over," he said. "Now arrives the hour of action."

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Analysis: Donald Trump Promises a Presidency Like No Other - NBCNews.com