The statement overlooked the fact that Mr. Trump himself urged Mr. Obama not to strike Syria when Mr. Assad crossed that red line. “President Obama, do not attack Syria,” Mr. Trump posted on Twitter at the time. “There is no upside and tremendous downside. Save your ‘powder’ for another (and more important) day!”

Still, Mr. Trump has been more consistent in making clear that he does not see it as the United States’ place to get involved in other countries’ affairs. During last year’s campaign, he posted on Twitter that “Syria is NOT our problem” and signaled acceptance of Russia’s annexation of Crimea from Ukraine. After taking office, when an interviewer suggested that President Vladimir V. Putin was a “killer,” Mr. Trump said the United States could hardly criticize. “What, you think our country’s so innocent?”

His foreign policy seems defined more by a transactional nationalism, rooted in the sense that the United States is getting ripped off — by NATO allies who are not paying enough for their own defense, by trading partners like China that are “eating our lunch” and by neighbors like Mexico that are sending drugs and criminals over the border. Rather than spreading American values, his policy aims to guard American interests.

“It struck me that it was very Chinese in orientation,” said Ian Bremmer, the founder and president of the Eurasia Group, a consultancy in Washington. “You take out all of the issues of American exceptionalism and values, you take out all the restraints and responsibilities of American alliances and architecture that are based on those values, and it creates a very different sense of foreign policy.”

Other presidents were not always consistent in speaking out for democracy and human rights, but they did make such issues a priority at times. Mr. Bush argued that decades of realism that subordinated democracy in favor of stability in the Middle East had yielded neither. In his second inaugural address, he vowed to predicate the United States’ relations with “every ruler and every nation” on their treatment of their own people. Mr. Obama thought Mr. Bush’s democracy push was too messianic, but he did encourage protesters during the Arab Spring and he did suspend some arms sales to Egypt after a military takeover.

Mr. Trump, by contrast, lifted human rights conditions on arms sales to Bahrain, a crucial American ally in the Middle East, and Secretary of State Rex W. Tillerson declined to personally present the State Department’s annual human rights report, as his predecessors did. Asked at a meeting on Tuesday with King Abdullah II of Jordan about the chemical weapons attack in Syria, Mr. Tillerson remained silent, only later issuing a written statement condemning it.