The pessimism is understandable. In the wake of President Trump's withdrawal from the Iran nuclear deal, the only people who seem happy are those who have long advocated for regime change and pre-emptive war, while those who have learned the lessons of Iraq are apoplectic or depressed. If the Iranians restart their nuclear program in response to the American pullout, and America or its regional allies respond with military force, the result would be an escalation of the many-sided Middle East conflict, with unpredictable consequences, few of them plausibly good.

But it's worth examining the more optimistic case. Not the notion that American pressure will suddenly bring about a better deal with Iran, where the regime abjures terrorism, shuts down its missile programs, opens its prisons, and allows fully free elections. After the abject failure of our confrontational policies in Cuba, North Korea, and elsewhere, it's very hard to credit anyone's continued belief that adversaries can simply be bullied into submission.

Nor is it especially likely that a democratic revolution will break out and sweep the country. The Iranian regime is extremely unpopular — but that regime is headed by someone mistrusted both by hardliners and liberal dissenters. The current popular unrest worries the regime precisely because it is more populist than liberal, and for that very reason it should give America's pro-democracy cheering section pause about prematurely cheering.

The optimistic case isn't the one advocates of withdrawal would make. But that doesn't mean it doesn't exist.

To start with, the Iranian regime has every incentive to salvage what remains of the Joint Comprehensive Plan of Action (J.C.P.O.A.), to which not only America but Britain, France, Germany, China, and Russia are party. A true nuclear program would be expensive and difficult to restart, and would hardly bring popular support if it led to further international isolation. The smart move for Iran would be to invite China, Russia, and Europeans to continue inspections and to press for international relief from American sanctions.

Similarly, the other parties have every incentive to try to salvage the deal, both commercial and geopolitical. All of these countries want to do business in Iran, and none of them want to see Iranian nuclearization. When interests are aligned, there's usually a deal to be made. If that deal excludes the United States, that's probably a win not only for Russia, China, and Iran, but arguably for our European allies. It's not like standing with the Trump administration is especially popular in London, Paris, or Berlin these days.

The main obstacle to such a maneuver's success would be American secondary sanctions on companies doing business in Iran. But the Chinese and Europeans may be able to call America's bluff, either by providing subsidies to any companies that are sanctioned by the Americans or by declaring America's secondary sanctions illegal and asking the World Trade Organization to arbitrate the dispute.

And if our trading partners retaliate against American companies, what would the Trump administration do when American CEOs complain that we're getting into a trade war over Iranian support for Hezbollah? Is it that implausible that Trump would back down and focus on personal sanctions against the Iranian leadership? If he did so, and abandoned secondary sanctions, the Iranians might wind up getting more international investment after an American pullout than they have in the two years since the deal was struck.

Meanwhile, if Iran didn't provide America with a clear case for military action, perhaps even hawkish National Security Adviser John Bolton would be deterred from indulging his predilection for diplomacy via bunker-busters.

What about America's credibility? A variety of observers have noted that the North Koreans, seeing how little an American agreement is worth, would be foolish to put any trust in a denuclearization deal.

But this may be backwards. The North Koreans never had any reason to trust the Americans. Perhaps the reason a deal is even possible now is that the North Koreans feel strong enough to make one, while the South Koreans see less and less value in their increasingly unreliable American ally. If pulling out of the Iran deal makes America look even weaker and less-reliable, that may increase the incentives for the two Koreas — and China — to come to agreement. And if they do, it's unlikely that Trump will pass up the chance to claim the credit — even if the terms give America almost none of what it has asked for. If nothing else, the North's decision to release three American prisoners doesn't sound like the behavior of a country looking to walk away from the table quite yet.

The true optimistic case on Trump's decision to pull out of the Iran deal is of a piece with the optimistic case for Trump in general. He certainly won't make America great again in the sense of restoring beneficent American hegemony. But by making it plain to the rest of the world that America is unreliable, he's accelerating the process by which the rest of the world works out arrangements that don't depend on American sponsorship. And, precisely because America remains extraordinarily powerful, it encourages them to do so without unduly provoking an American military response.

America may bluster, but quietly and in the background, the rest of the world may just be moving on.