David Sanger

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.

Secretary of State John Kerry and his Russian counterpart, Sergey V. Lavrov, announced that they had agreed on the delivery of desperately needed aid to besieged Syrian cities, to be followed by a “cessation of hostilities” on the way to a more formal cease-fire.

International inspectors confirmed Saturday that Iran had dismantled large sections of its nuclear program, as agreed in a historic agreement last summer, paving the way for the lifting of oil and financial sanctions by the United States.

A crisis over the seizing of two U.S. patrol boats in the Persian Gulf was averted Wednesday when Iran returned the craft and released their crews as Pentagon officials struggled to explain how the boats had ended up near a major Iranian naval base.

Two U.S. Navy patrol boats and their crews were seized by the Iranian authorities in the Persian Gulf on Tuesday during what a Tehran news agency alleged was “snooping.”

The Energy Department and the Pentagon have been readying a weapon with a build-it-smaller approach, setting off a philosophical clash in the world of nuclear arms.

The detonation that rocked northeast Asia and Pyongyang’s claim to have set off its first hydrogen bomb — a boast there is good reason to treat with skepticism — is a reminder that the North Koreans have been on something of an atomic spending spree while U.S. negotiators were striking deals with the Iranians.

The agreement represented the first time all the major outside participants in the conflict now in its fifth year had agreed on the start of a political process to bring it to an end.

Russian submarines and spy ships are aggressively operating near the vital undersea cables that carry almost all global Internet communications, raising concerns that they may plan to attack those lines in times of tension or conflict.

Iran and the world powers said here Thursday that they had reached a surprisingly specific and comprehensive general understanding about the next steps in limiting Tehran’s nuclear program, though Western officials said many details needed to be resolved before a final agreement in June.

If U.S. negotiators are ultimately able to conclude a “political understanding” with Iran on its nuclear program, as they said they were striving to do Thursday morning, the seeds might have been planted earlier in the week.

The one-day extension came with a White House warning that the Obama administration was prepared to step back from its diplomatic efforts if it concluded that the initial accord could not be reached.

With a negotiating deadline just two days away, Iranian officials on Sunday backed away from a critical element of a proposed nuclear agreement, saying they are no longer willing to ship their atomic fuel out of the country.

Stepping into a heated debate within the nation's intelligence agencies, President Barack Obama has decided that when the National Security Agency discovers major flaws in Internet security, it should - in most circumstances - reveal them to assure that they will be fixed.