The deal with Turkey to pull U.S. troops out of Syria is a typical Trumpian mess, with rash, poorly planned presidential action leading to pernicious — and downright bloody — consequences. Yet that initiative also represents an effort, badly executed and communicated, to bring about a paradigm shift in America’s war on terror.

U.S. President Donald Trump is trying to usher in a fourth phase of that post-9/11 conflict, in which the United States would accept greater security risk as the price of reducing the ongoing costs of involvement in the greater Middle East. He is running head-on into opposition from many in his own party, who are still more inclined to pay higher costs to buy down the threat of terrorist attacks. Trump is so far getting the worst of the debate. But the underlying issue he has raised is not going away anytime soon.

Since the Sept. 11 attacks, the U.S. struggle against terrorism has gone through three phases. The first phase was the “anywhere, anytime” approach taken by the George W. Bush administration in the years following 9/11. The U.S. mounted large-scale invasions and long-term nation-building projects in Iraq and Afghanistan. The possibility of devastating follow-on attacks seemed unacceptably high, so U.S. officials were willing to pay quite a price to suppress terrorist groups, defeat state sponsors, and attempt to transform the conditions that produced violent extremism.

The frustrations of this approach — particularly the botched and costly occupation of Iraq — eventually led to a second phase of the war on terrorism. The Barack Obama administration emphasized lighter-footprint operations using drones and special operations forces, and it withdrew U.S. troops from Iraq, the war the president had made his reputation opposing. After initially — and reluctantly — surging more U.S. troops into Afghanistan, the Obama administration also began to wind down the American presence there after 2011.

This strategy seemed effective for a time, but allowed the threat to come back with a vengeance, with rise of the Islamic State across Syria and Iraq in 2013-2014.

This setback led to a third phase of the fight against global terrorism, which began in the final innings of the Obama presidency and continued, for a time, under Trump. Having seen that relaxing the pressure on the enemy could backfire — and with the Islamic State and its sympathizers having perpetrated major attacks in Europe and the U.S. — American officials sought a middle ground between the two earlier approaches. This became a medium-footprint approach that employed modest numbers of ground troops, while also using air power, logistics, intelligence and other enablers to support local partners in the fight against IS.

Trump is now seeking to shift U.S. strategy once again. The president has no objection to smashing terrorist organizations that are suspected of plotting attacks against the U.S. Yet he seems desperate to end the post-conflict stability operations that tend to follow even medium-footprint operations like the one in Syria. The U.S. should “ONLY FIGHT TO WIN,” he tweeted on Oct. 7, after American forces began to pull back from the Syria-Turkey border. If the threat returns, the U.S. can again apply overwhelming force: “We are 7000 miles away and will crush ISIS again if they come anywhere near us!”

There are a number of obvious problems with what the president is proposing. By paving the way for a Turkish invasion of northern Syria, the U.S. pullback is likely to distract and weaken the Kurdish-led Syrian Democratic Forces — America’s principal ally there — and thereby make easier an IS resurgence. Trump’s policy has the smell of abandoning the Kurds to the mercies of an autocratic Turkish regime that is frequently hostile to American policy objectives. U.S. retrenchment may also set off a scramble for influence in northern Syria, empowering Russia, Iran and other bad actors.

For all these reasons, the president’s decision earned harsh rebukes in Washington: Sen. Lindsey Graham, who had backed Trump strongly since the 2016 election, called the pullback “the biggest mistake of his presidency.” Yet when one cuts through the specific critiques of Trump’s policy, the basic dispute comes back to a familiar issue: competing assessments of cost and risk.

Trump is determined to drive down the material price of U.S. involvement in the Middle East, a region he sees as having little strategic importance. So he is willing to accept a greater risk that groups such as IS will not stay defeated over the long run. “It is time for us to get out of these ridiculous Endless Wars, many of them tribal, and bring our soldiers home,” he wrote in the Oct. 7 tweet storm. If IS does stage a comeback, he added, perhaps it will target Europe before it targets America.

Yet most Republican leaders, apart from dovish or non-interventionist outliers like Sen. Rand Paul, are using a different calculus. Graham, Senate Majority Leader Mitch McConnell and others in the GOP caucus seem more inclined to hedge against the risks of an IS resurgence, even if that means bearing higher monetary and human costs in the meantime.

A U.S. withdrawal “would increase the risk that ISIS and other terrorist groups regroup,” McConnell noted in a rare public critique of the president. He warned that Trump is in danger of repeating Obama’s error in pulling back prematurely from Iraq and then having to intervene once again after the American position has deteriorated badly. Political and security concerns work hand-in-hand here: If the U.S. withdraws from Syria or Afghanistan and the result is a major terrorist attack in Europe or America, Republicans risk gifting Democrats the ability to brand them as being soft on terror.

So far, Trump is struggling to shift U.S. strategy and the calculations underlying it. Opinion polls show that Americans, Republicans especially, still place terrorism at or near the top of the list of security threats facing the country. In gesturing toward retrenchment in the Middle East, the president has run into not only a firestorm of criticism from his own party but also resistance from within his own administration.

The last time he threatened to pull troops out of Syria, in December, his secretary of defense, Jim Mattis, resigned. The president may still insist on full withdrawals from Syria and Afghanistan, but he will have to pay a political price within the Republican Party to do so.

Yet even if Trump doesn’t succeed in ending the forever wars, the basic question he has raised will persist. He has had little success moving the Republican Party on terrorism-related issues, but many Democrats — including some of the party’s leading presidential candidates — have also called for ending America’s wars in the Middle East. The pull of competing priorities in Europe and the Indo-Pacific will only become stronger in the coming years. The U.S. will increasingly be torn between its desire to be done with frustrating wars in the Middle East and its reasonable fears about what will happen after its troops depart.

Trump, no doubt, is one of a kind, and he deserves no praise for a policy that has helped bring chaos to one of the few relatively stable areas of Syria. But the debate he has set off over how to balance cost and risk in the war on terrorism will shake up U.S. policy for years to come.

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. Most recently, he is the co-author of “The Lessons of Tragedy: Statecraft and World Order.”

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