Mr. Buttigieg, whose political experience is limited to two terms as mayor of a midsize city, also repeatedly invoked his military service to suggest a battle-tested heft on the global stage. He was a Navy Reserve officer deployed in Afghanistan for seven months in 2014, midway through his first term. He described himself as an American forged by 9/11, when “war came to our generation.”

Early in his campaign, Mr. Buttigieg was criticized for introducing himself based on personality, rather than policy, and his address Tuesday, at Indiana University in Bloomington, was his most substantive policy speech to date. Nonetheless, he said, “I do not aspire to deliver a full Buttigieg Doctrine today.”

But he went on to offer prescriptions on a broad range of international issues.

He said the “mission drifted” in Iraq and Afghanistan, at a cost of great “moral authority” to the United States, and he called for Congress to repeal its authorization for the president to wage war in those countries. “I fear that someday soon we may receive news of the first U.S. casualty of the 9/11 wars who was born after 9/11,” Mr. Buttigieg said.

Throughout the nearly hourlong speech, Mr. Buttigieg launched broadsides at the president, though not by name. Russia, he said, should be viewed “not as a real estate opportunity” but as an adversary that disrupted an American election. On North Korea, he promised, “You will not see me exchanging love letters” with its brutal dictator. China poses far more of a threat than “the export-import balance on dishwashers,” he said, ripping Mr. Trump’s obsession with tariffs and the trade balance.

Mr. Buttigieg drew a picture of a more internationally connected United States than Mr. Trump’s America First policies, which have questioned longstanding security alliances and sought to stem the tide of globalization through tariffs.