And he said he would be willing to withdraw United States forces from both Japan and South Korea if they did not substantially increase their contributions to the costs of housing and feeding those troops. “Not happily, but the answer is yes,” he said.

Mr. Trump also said he would seek to renegotiate many fundamental treaties with American allies, possibly including a 56-year-old security pact with Japan, which he described as one-sided.

In Mr. Trump’s worldview, the United States has become a diluted power, and the main mechanism by which he would re-establish its central role in the world is economic bargaining. He approached almost every current international conflict through the prism of a negotiation, even when he was imprecise about the strategic goals he sought. He again faulted the Obama administration’s handling of the negotiations with Iran last year — “It would have been so much better if they had walked away a few times,” he said — but offered only one new idea about how he would change its content: Ban Iran’s trade with North Korea.

Mr. Trump struck similar themes when he discussed the future of NATO, which he called “unfair, economically, to us,” and said he was open to an alternative organization focused on counterterrorism. He argued that the best way to halt China’s placement of military airfields and antiaircraft batteries on reclaimed islands in the South China Sea was to threaten its access to American markets.