As a result, the Iraqi Kurds created an autonomous zone in the northern part of the country, which has provided a valuable element of stability for more than two decades and was indispensable in driving ISIS out of Iraq and Syria. As Provide Comfort approached the end of its three‑month authorization, American military commanders proposed removing the NATO ground force. As under secretary of defense at the time, I objected — that could have opened the door for Iraqi tanks to return and resume massacring the Kurds. So I was sent to Iraq to see how the operation was working. There, I saw how that light NATO ground force had liberated a large portion of northern Iraq from Mr. Hussein’s grasp, thanks to overpowering air support. Secretary of Defense Dick Cheney then authorized the withdrawal of the ground component but retained the American air cover over the Kurdish fighters.

In contrast, the American failure to support a Shiite uprising the previous month in southern Iraq is a mistake that we still suffer from today.

In March of 1991, following Mr. Hussein’s defeat in Kuwait, the Shiites of southern Iraq rose up against the dictator. Although the United States originally encouraged an uprising, it then abandoned the rebels to the savagery of Mr. Hussein’s tanks and chemical weapons. At no risk to American lives, and without moving even one yard closer to Baghdad, the United States could have stopped those tanks from moving to crush the rebellion and stopped the helicopters from flying and dropping chemical weapons.

That American inaction enabled Saddam Hussein to slaughter tens of thousands of Iraqi Shiites, whose bodies were discovered in mass graves 12 years later , and led to a much costlier, longer second war. We would never again have such a good opportunity to remove Mr. Hussein from power without risking American lives and without going to Baghdad.

Similarly, President Obama’s failure 20 years later to support the Syrian opposition, when that support might have toppled the regime of President Bashar al-Assad with little risk to Americans, had both moral and strategic consequences. The eight blood‑soaked years that the Assad regime has remained in power may have cost more than half a million Syrian lives and has created hundreds of thousands of refugees and internally displaced people. That humanitarian disaster also produced the strategic vacuum from which ISIS emerged in northern Syria to invade and destabilize Iraq, forcing Mr. Obama to return the troops he had withdrawn just a few years earlier. Now with President Trump building on that earlier failure, Russia and Iran may gain effective control of Syria.

Mr. Trump should have told President Erdogan that Turkey had helped to create the ISIS problem in Syria, that it should now “back off” and let negotiations continue and that sending in the Turkish Army would be something Mr. Erdogan would regret. Instead of conceding that Syria’s Kurds are a threat to Turkey, he might have suggested using the considerable American influence over the Kurds to prevent that.

To Mr. Trump’s credit, he responded to General Keane’s criticism by inviting him to the White House. There, the general pointed out the strategic consequences of allowing Iran’s proxies to gain control of the oil resources of northeastern Syria. This has given Mr. Trump an opportunity not to undo his decision — he has unfortunately already created a new and much more complicated situation — but to revise it and continue some support for our Kurdish and Arab allies so that they can achieve a reasonable negotiated settlement.

The goal of a revised operation should be made clear: It is not to seize Syria’s oil, as Mr. Trump has suggested, but rather to keep that strategic asset out of the hands of our enemies.

Mr. Wolfowitz served in the administrations of George H.W. Bush and George W. Bush. He is a visiting scholar at the American Enterprise Institute.

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