In Donald Trump's Worldview, America Comes First, and Everybody Else Pays

"If Saudi Arabia was without the cloak of American protection," Mr. Trump said during a 100-minute interview on foreign policy, spread over two phone calls on Friday, "I don't think it would be around."

He also said he would be open to allowing Japan and South Korea to build their own nuclear arsenals rather than depend on the American nuclear umbrella for their protection against North Korea and China. If the United States "keeps on its path, its current path of weakness, they're going to want to have that anyway, with or without me discussing it," Mr. Trump said.

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.

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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.

"We have tremendous economic power over China," he argued. "And that's the power of trade." He did not mention Beijing's ability for economic retaliation.

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Pressed about his call to "take the oil" controlled by the Islamic State in the Middle East, Mr. Trump acknowledged that this would require deploying ground troops, something he does not favor. "We should've taken it, and we would've had it," he said, referring to the years in which the United States occupied Iraq. "Now we have to destroy the oil."

He did not rule out spying on American allies, including leaders like Angela Merkel, the German chancellor, whose cellphone was apparently a target of the National Security Agency. Mr. Obama said the agency would no longer target her phone but made no such commitments about the rest of Germany, or Europe.

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"Not isolationist, but I am America First," he said. "I like the expression." He said he was willing to reconsider traditional American alliances if partners were not willing to pay, in cash or troop commitments, for the presence of American forces around the world. "We will not be ripped off anymore," he said.