The capture of the tanker is a sharp step up after three months of rising tensions between Iran and the West that last month brought the United States within minutes of a military strike against targets in Iran. A fifth of the world’s crude oil supply is shipped from the Persian Gulf through the narrow Strait of Hormuz off the coast of Iran, and oil prices spiked sharply on Friday even before the British warning.

But the crisis has also caught Britain at a singularly vulnerable moment, with Prime Minister Theresa May expected to resign on Wednesday. As it seeks to craft a response to the seizure, the British government has been all but paralyzed by a leadership contest withing the governing Conservative Party to determine her successor.

What’s more, the favorite in the leadership race, Boris Johnson, a former foreign minister, is famously unpredictable. He has said during his campaign that he stands with the other European powers in their desire to avoid a confrontation with Iran. But Mr. Johnson has also sought closer ties to President Trump, who set the current cycle of confrontation in motion by attempting to squeeze Iran into renegotiating the 2015 nuclear accord.