What does the sad story of Jamal Khashoggi—the Saudi journalist who went into the kingdom’s consulate in Istanbul and never came out—have to do with debates about the US policy in the Middle East? Quite a lot.

For those who think Washington can simply turn over the management of Middle Eastern geopolitics to the countries of the region, Khashoggi’s disappearance is one more reminder that things are not so simple. A post-American Middle East will not be stable and peaceful. It will be even nastier and more turbulent than it is today.

The basic argument to pull back from the Middle East runs like this: The US should not exert such vast energies to confront challenges like terrorism and Iranian expansionism, because the countries of the region should and can do it themselves.

The only way to correct this situation is through retrenchment. In the more moderate formulation, retrenchment means simply withdrawing US ground forces and swearing off any significant use of American military power. In the more radical formulation, it might entail liquidating the entire US military presence, including naval forces, and exercising less diplomatic influence as well.

US President Donald Trump, while a staunch supporter of the Arab states in their rivalry with Iran, has repeatedly demanded that they take more responsibility so the US can take less.

The desire to get more out of US allies and partners is sensible enough, as is the notion that the US cannot fight full-scale counter-insurgencies in the Middle East forever. Yet the idea that Washington can simply hand off the responsibility for the Mideast’s regional order rests on a fatally flawed assumption: that these allies will behave as responsibly and competently as the US wants them to behave after it has largely left the region.

To see why this assumption is so flawed, just look at the recent behaviour of Saudi Arabia. It is by far the richest state in the region with by far the largest military budget—third largest in the world by some estimates. It already plays an important role in Middle Eastern geopolitics; it could and probably would play a far larger role were the US less involved in the region’s affairs. Yet that prospect is not reassuring, because Saudi conduct since 2015 has been destabilizing in the extreme.

In March of that year, the Saudis responded to a real but manageable security threat—the takeover of Yemen by Iranian-supported Houthi rebels—with a poorly planned and executed invasion. The war has had not only catastrophic humanitarian effects, but also led to increased Iranian influence in Yemen.

In June 2017, Riyadh engineered a diplomatic showdown with Qatar, meant to make that small country a vassal state. The showdown backfired, causing a rift with the state department and Pentagon—if not the White House—and leading Qatar to deepen its ties to Iran and Turkey. That November, the Saudi government effectively kidnapped Lebanon’s prime minister, Saad Hariri, in a dispute over Iranian influence in his country. That gambit, too, backfired, further destabilizing Lebanon and provoking international condemnation.

And last week the Saudi security services reportedly detained—and allegedly murdered—Khashoggi, a fierce critic of the current government, led by crown prince Mohammad bin Salman (MBS). If the allegations are true, the Saudi regime carried out an extrajudicial killing of an internationally recognized figure in a way that is sure to infuriate Turkey, another regional power.

Much of Saudi Arabia’s recent behaviour has been linked to the rise of MBS, who seems driven by a combination of ambition, arrogance and recklessness. Yet it is not a coincidence that Saudi misdeeds have accumulated at a time when the US is widely seen to be drawing down in the Middle East.

The Saudi invasion of Yemen, for instance, seems to have been motivated by a perception that the Barack Obama administration was no longer committed to containing Iran, so the kingdom would have to do that job itself. The confrontation with Qatar came as the Trump administration—or at least the Trump family—signalled that it was giving Saudi Arabia free rein and retreating from the traditional US role of suppressing, rather than encouraging, fights between its friends.

As the US has pulled back modestly, the Saudis have indeed rushed forward, with mostly lamentable results.

This last point touches on one of the dirty secrets of the role of the US in the Middle East and other key regions. The US maintains a presence not simply to deter competitors such as Iran, Russia and China. It also manages conflicts between allies—whether Japan and South Korea in East Asia, or Saudi Arabia and its Gulf neighbours—and steers them away from dangerous behaviour.

Yet this approach only works if the US is present and committed. If it retreats from the Middle East, it will lose whatever restraining leverage it once had over allies and competitors alike. It will leave behind not tranquillity, but a more chaotic, rivalrous environment in which other nations feel forced to fend for themselves.

The US is entering a period in which its national security resources will be stretched thin, and there will be continuing calls to withdraw from a region that has been the source of such trouble. But those who advocate retrenchment need to be honest about what will follow: a Middle East even more dangerous than the one we know now. BLOOMBERG VIEW.

Hal Brands is a Bloomberg Opinion columnist.