The Democratic Party’s progressive presidential candidates want to end the “forever war” — America’s two-decade struggle against jihadi extremism. The trouble is that they don’t know how.

It is easy enough for the progressives to argue that the United States should pull back from the greater Middle East and demilitarize its counterterrorism strategy. Unfortunately, they have less to say about how the U.S. can do so responsibly.

The idea that the forever war must end has become a consensus position on the progressive left. Last November, Sen. Elizabeth Warren kicked off the 2020 foreign policy sweepstakes with a speech that condemned the war on terrorism as costly, counterproductive and morally debasing. She called for the U.S. to conclude a peace deal with the Taliban so that American forces can withdraw from Afghanistan. Likewise, Rep. Tulsi Gabbard (a combat veteran whose views blend progressivism with an array of other influences) has consistently criticized America’s policies in the Middle East and advocated greater restraint.

Most recently, Sen. Bernie Sanders authored an essay in Foreign Affairs on the subject of “ending America’s endless war.” Sanders argued that the war on terrorism has been a “disaster” and that America must “rethink the militaristic approach that has undermined the United States’ moral authority, caused allies to question our ability to lead, drained our tax coffers, and corroded our own democracy.”

The U.S., Sanders believes, must refocus on the issues that truly matter in the 21st century — providing greater social justice and broadly shared economic opportunity at home, dealing with climate change and resisting the rise of revisionist authoritarian powers. And doing so requires withdrawing from Afghanistan, ending U.S. support for the Saudi-led war in Yemen and getting away from the military-first approach to counterterrorism.

At first blush, some of this seems reasonable enough. There has been a frustrating, whack-a-mole quality to the war on terrorism: When the U.S. crushes one enemy, an even nastier one takes its place. There are more important issues the nation must address today, particularly the threat that China and Russia might fracture the historically peaceful, prosperous and democratic international order Washington and its allies have created.

Sanders and others are also right in saying that America should never again engage in torture, and that the U.S. must be discriminating in how it defends itself from terrorism. The trouble is that, so far, the progressives have not answered the tough question about how America can safely pull back from the war on terror.

Consider Afghanistan. Sanders is clear that he wants the U.S. to pull out its troops. But al-Qaida and Islamic State are still active there, and the U.S. intelligence community has reportedly warned that a complete U.S. withdrawal could lead to a major terrorist attack on American soil within two years.

So how would Sanders ensure that a withdrawal doesn’t simply create a dangerous vacuum that vicious killers will exploit? His answer is vague at best: “We will work closely with our partners and allies to design a serious diplomatic and political strategy to stabilize the region, promote more effective and accountable governance and ensure that threats do not re-emerge after we leave.” If it was that easy, American troops would have left Afghanistan many years ago.

Or think about the U.S. role in Yemen. Progressives — as well as most Democrats and some Republicans — want to cut off American support to the Saudi-led coalition. That’s entirely understandable, given that the war has produced appalling humanitarian consequences without achieving much in the way of strategic success.

And there is a good argument to be made that the U.S. should more aggressively use the leverage its support provides — with both Saudi Arabia and its Houthi adversaries — to push for a diplomatic settlement in Yemen. Yet there is no guarantee that simply terminating that support will make a horrific situation better rather than worse. If a U.S. pullback does not force Saudi Arabia to end the war, for instance, the upshot might be to make the Saudis even less discriminating and effective in waging the conflict.

Finally, there is the troop presence in Iraq and Syria. It is fair to say that the George W. Bush administration never should have invaded Iraq in the first place. It is correct to argue, as Sanders and other progressives do, that the U.S. shouldn’t start a war against Iran today. But this isn’t much help in thinking about whether Washington should leave a few thousand personnel in Iraq and Syria to keep the American boot on Islamic State’s back and prevent a replay of what happened in 2013-2014, when a nearly defeated al-Qaida in Iraq morphed into the IS juggernaut that rolled across a large swath of the Middle East.

This speaks to a broader problem with the progressive critique. Sanders tends to take aim at the version of the war on terrorism America was fighting in 2009, not the version it is fighting today. Then, the U.S. had nearly 200,000 troops doing counterinsurgency in Iraq and Afghanistan. Now, it has far smaller contingents scattered across the greater Middle East and North Africa. Washington often relies on “by, with and through” strategies in which modest numbers of U.S. forces support local troops who do most of the fighting and dying. There are plenty of problems with this strategy. But the fact is that the U.S. already has moved away from the large-scale interventions the progressives oppose, in favor of a more calibrated approach.

The biggest shortcoming of the progressive critique is that it elides the hard strategic choices about where to go from here. The real question is not whether the U.S. should simply walk away from frustrating military engagements. For if we do this, we are likely to discover — once again — that the problems of the greater Middle East do not stay there, that they can become far more dangerous when America is not around to keep the lid on, and that a superpower that decides to get out of the region will ultimately find itself pulled back in, under worse circumstances than before.

The real questions, rather, are more nuanced. They involve considering to what extent the U.S. can responsibly reduce its military presence and geopolitical focus in the region, which terrorist groups can safely be ignored or left to indigenous forces, and how much additional risk of a major homeland attack the U.S. should accept as the price of focusing on other issues.

America could profit tremendously from a serious, analytical debate on these questions. It won’t get much benefit from polemical critiques that dodge the truly tough questions about how to handle the forever war in the future.

Hal Brands is a Bloomberg Opinion columnist, the Henry Kissinger distinguished professor at Johns Hopkins University’s School of Advanced International Studies and a scholar at the American Enterprise Institute.

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