From David E. Sanger and Maggie Haberman writing in the New York Times:

Donald J. 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,” 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.

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.