"If this isn't checkmate, it's close," argues Kagan. "In recent days, Trump has reportedly asked the U.S. intelligence community to assess the consequences of simply declaring victory and walking away."

You can't blame him. Hoping for regime collapse is not much of a strategy, especially when the regime has already survived repeated military and economic pummeling. It could fall tomorrow, or six months from now, or not at all. Trump doesn't have that much time to wait, as oil climbs toward $150 or even $200 a barrel, inflation rises, and global food and other commodity shortages kick in. He needs a faster resolution.

But any resolution other than America's effective surrender holds enormous risks that Trump has not so far been willing to take. Those who glibly call on Trump to "finish the job" rarely acknowledge the costs. Unless the U.S. is prepared to engage in a full-scale ground and naval war to remove the current Iranian regime, and then to occupy Iran until a new government can take hold; unless it is prepared to risk the loss of warships convoying tankers through a contested strait; unless it is prepared to accept the devastating long-term damage to the region's productive capacities likely to result from Iranian retaliation"walking away now could seem like the least bad option. As a political matter, Trump may well feel he has a better chance of riding out defeat than of surviving a much larger, longer, and more expensive war that could still end in failure.