Donald Trump, the Republican presidential front-runner, said that if elected, he might halt purchases of oil from Saudi Arabia and other Arab allies unless they commit ground troops to the fight against the Islamic State or "substantially reimburse" the United States for combating the militant group, which threatens their stability.

"If Saudi Arabia was without the cloak of American protection," Trump said during a 100-minute interview on foreign policy, spread over two phone calls, "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 U.S. 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," Trump said.

And he said he would be willing to withdraw U.S. forces from both Japan and South Korea if the countries do not substantially increase their contributions to the costs of housing and feeding those troops.

"Not happily, but the answer is yes," he said.

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

Trump said he believes 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 President Barack Obama's administration on the 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.

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 anti-aircraft batteries on reclaimed islands in the South China Sea was to threaten its access to U.S. markets.

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

Trump's views, as he explained them, fit nowhere into the recent history of the Republican Party: He is not in the internationalist camp of President George H.W. Bush, nor does he favor George W. Bush's call to make it the United States' mission to spread democracy around the world. He agreed with a suggestion that his ideas might best be summed up as "America first."

"Not isolationist, but I am America first," he said. "I like the expression."

Much the same way he treats political rivals and interviewers, he personalized how he would engage foreign nations, suggesting his approach would depend partly on "how friendly they've been toward us," not just on national interests or alliances.

At no point did he express any belief that U.S. forces deployed on military bases around the world were by themselves valuable to the United States, though Republican and Democratic administrations have for decades argued that they are essential to deterring military adventurism, protecting commerce and gathering intelligence.

Like Richard Nixon, Trump emphasized the importance of "unpredictability" for a U.S. president, arguing that the country's traditions of democracy and openness had made its actions too easy for adversaries and allies alike to foresee.

"I wouldn't want them to know what my real thinking is," he said of how far he was willing to take the confrontation over the islands in the South China Sea, which are remote and uninhabited but extend China's control over a major maritime thoroughfare. But, he added, "I would use trade, absolutely, as a bargaining chip."

Until recently, Trump's foreign-policy pronouncements have largely come through slogans: "Take the oil," "Build a wall" and ban Muslim immigrants, at least temporarily. But as he has pulled closer to capturing the nomination, he has been called on to elaborate.

Pressed about his call to "take the oil" controlled by the Islamic State in the Middle East, 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 U.S. allies, including leaders like Angela Merkel, the German chancellor, whose cellphone was apparently a target of the National Security Agency. Obama said the United States would no longer target her phone but made no such commitments about the rest of Germany, or Europe.

"I'm not sure that I would want to be talking about that," Trump said. "You understand what I mean by that."

In criticizing the Iran nuclear deal, Trump expressed particular indignation at how the roughly $150 billion released to Iran -- by his estimate; the number is in dispute -- was being spent.

"Did you notice they're buying from everybody but the United States?" he said.

Told that sanctions under United States law still bar most U.S. companies from doing business with Iran, Trump said: "So, how stupid is that? We give them the money and we now say, 'Go buy Airbus instead of Boeing,' right?"

But Trump, who has been pushed to demonstrate a basic command of international affairs, insisted that voters should not doubt his foreign-policy fluency.

"I do know my subject," he said.

A Section on 03/27/2016