Michael O'Hanlon and Omer Taspinar

Opinion contributors

President Donald Trump is pulling U.S. forces out of at least two of the 15 or so positions they now maintain in northeastern Syria, where Kurdish-dominated forces of the Syrian Democratic Front control most territory and hold more than 10,000 ISIS-affiliated prisoners in detention. His purpose is apparently to free up the border area for the Turkish incursion that has just been reported.

Ironically, Trump also simultaneously threatens to retaliate against economically against Turkey, should Turkey's President Erdogan decide that going after Kurdish terrorists on his soil requires such a cross-border invasion against other Kurds in the region. Trump also claims, in his various tweets and comments of the last couple of days, that pulling the 1,000 U.S. forces who now remain in Syria out of that country would honor a campaign promise to wind down America's overseas military commitments. Perhaps the United States will soon leave all of its positions inside Syria; it is hard to know what the president intends.

Should President Trump keep his 'campaign promises?'

This logic is badly flawed. There are several reasons why no political promise or electoral mandate requires any such withdrawal — whether from the two positions near the Turkish border, or the other parts of Syria where GIs help keep an eye on ISIS, Iran, and other threats:

► Politicians claim mandates all the time but rarely is the verdict of an election so clear as to provide them. We do not vote issue-by-issue for candidates, with multiple-part voter referenda on many issues. We ultimately choose one candidate, who has inevitably made dozens of promises in the course of a campaign, over another who has made her own multiple pledges. As such, we cannot know which promises mattered most to voters. Polls can inquire, but they are rarely clear or conclusive. This point is obvious at one level, yet elected leaders (not just Trump) repeatedly invoke supposed mandates to justify their own gut instincts or policy preferences when in fact they can lay claim to no such thing. Arguably, Trump's promises about economic nationalism, views on law and order, opposition to several of President Obama's policies on matters like health care and energy, Supreme Court preferences, and several other matters all gained more attention and traction in his campaign than the promise to reduce military commitments abroad. Does he really have a mandate about all these things?

Utter confusion:Biggest danger of Syria withdrawal is Trump's lack of a plan

► The current U.S. military presence in Syria of 1,000 troops represents 0.05 percent of all American military forces in uniform today. That's right, to be crystal clear, that figure is one-twentieth of one percent of American military strength, since the United States has more than two million total troops, active and reserve. Put differently, it is roughly 0.5 percent of all U.S. forces deployed or based abroad today. In terms of costs, a rough estimate would also suggest that those 1,000 troops backed up by American airpower in places like Qatar consume less than 0.5 percent of the total defense budget. These costs and strains are entirely sustainable.

► That total of 1,000 troops is also less than the figure of roughly 2,000 in Syria that Trump inherited from Obama nearly three years ago. Why not view that as vindication of his campaign promise to reduce U.S. forces abroad, rather than feel any need to aim for zero? Did voters really expect or demand that latter outcome? In fact, polls suggest that issues like Syria and Afghanistan were nowhere near the top of most voters' priorities back in 2016, or today.

Why does total withdrawal have to be the answer?

► President Trump can take some justifiable pride that, building on the mission inherited from President Obama, his strategy has defeated the ISIS caliphate in Syria and Iraq. That counterterrorism objective was arguably his larger campaign promise, rather than the one about pulling forces home, and he has achieved it. But it is a fragile accomplishment, since the ISIS fighters were not all killed, and many of those who were captured, as noted above, are now being guarded by Kurdish forces in the very part of Syria that Erdogan now threatens to attack. The Kurds will necessarily defend themselves and their communities if that eventuality comes to pass, meaning that they will have fewer personnel to guard ISIS fighters (some of whom already occasionally slip out of the camps/prisons where they now live). More unreformed extremists will inevitably escape, repudiating Trump's pledge to squelch terrorism throughout the region.

At this point, the Syrian Kurds have considerable leverage on the ground because they are holding 12,000 ISIS prisoners in several detention facilities. Moreover, there are some 58,000 ISIS family members at the al-Hol camp in northeastern Syria. President Erdogan has apparently promised his army can take care of this problem. Yet, it is clear that the Turkish military will have its hand full fighting its main target. ISIS has never been the top priority of the Turkish national security establishment. It should surprise no one if the Kurds release the ISIS prisoners and prove that Turkey has in fact no power to control the real terrorist threat in Syria.

We've done our job by defeating ISIS:Trump is right to withdraw US troops from Syria.

► Thus, Trump has conflicting campaign promises, it would appear — defeat ISIS and al Qaeda on the one hand, bring back U.S. troops from frustrating and long deployments abroad on the other. How to resolve such a contradiction? How about appealing, not only to common sense given the small size and considerable effectiveness of the American presence in Syria today, but to the Constitution that Trump was elected to uphold? That Constitution makes him commander in chief and charges the federal government with national defense as its first and paramount priority. In a situation where American GIs are successfully defending the nation's security interests abroad, in an economical and efficient manner, the onus should be on advocates any option that would weaken or end such a capability.

President Trump's frustration with the forever wars is understandable. But in Syria, we have after years of policy failure, heartbreak, and danger to western Europe and North America, found an effective policy that involves very modest costs to the United States in blood and treasure. President Trump should celebrate and preserve this success rather than feel some need to end it.

Michael O'Hanlon, a senior fellow at The Brookings Institution and author of "The Science of War: Defense Budgeting, Military Technology, Logistics, and Combat Outcomes" is a member of USA TODAY's Board of Contributors. Follow him on Twitter: @MichaelEOHanlon. Omer Taspinar is a scholar at the Brookings Institution and a professor at the National War College.