In addition, the Persian Gulf monarchies that America sees as its containment front line are not united against Iran. Saudi Arabia and United Arab Emirates have been picking fights with Qatar, which has opened fissures among the Gulf’s monarchies, divided the region’s Sunnis and undermined America’s hope that they will unite against Shiite Iran. The Gulf Cooperation Council, which America helped create as a regional bulwark against Iran, now exists in name only. The council still holds fast to a Saudi-led blockade of Qatar, leaving Oman and Kuwait to see Saudi Arabia, rather than Iran, as the more urgent threat. For Qatar, Oman and Kuwait, Iran is a counterweight needed to protect them from Saudi heavy-handedness.

The monarchies — Saudi Arabia, Qatar, Oman, Kuwait and the Emirates — can offer America military bases, but as a recent drone attack on Saudi oil facilities made abundantly clear, the monarchies on their own are no match for Iran.

Furthermore, the monarchies, worried about the cost of war to their economies, have started to hedge their bets. The U.A.E. has opened channels of communication with Iran in order to ease tensions. According to the prime minister of Iraq, Adel Abdul Mahdi, when Maj. Gen. Qassim Suleimani, the head of the Quds Force in Iran’s Revolutionary Guards, was killed in Iraq by an American drone, he was carrying Iran’s response to a message Saudi Arabia had sent through Mr. Mahdi. The Arab monarchies will support American pressure on Iran, but they will also look to de-escalate their own tensions with Tehran — puncturing holes in whatever containment Americans envision.

Iran, too, is not the same challenge that American strategists grappled with during the heyday of containment. It is true that Iran’s revolutionary ideology now rings hollow at home, where Iranians are tired of repression, economic hardship and international isolation. That is why they embraced the 2015 nuclear deal as the beginning of the end of the old order. More recently, they have taken to the streets, demanding change. That may have convinced Washington that the Islamic republic is teetering, and that more pressure could topple it.