This photo released on Monday, Sept. 10, 2018 by the Syrian Civil Defense group known as the White Helmets, shows civil defense workers clean rubble from a house which was damaged by a Syrian government airstrike, in Hobeit village, near Idlib, Syria.  (Syrian Civil Defense White Helmets via AP)

Some foreign policy controversies are all-consuming in the moment but soon fade into history; others loom larger as time passes. The Syria “red line” incident of 2013 falls into the latter category. The fifth anniversary of that episode is upon us, at a time when fears of chemical weapons attacks by Bashar al-Assad’s Syrian regime — and a potential U.S. military response — are again rising.

The original incident unleashed vitriolic debates about coercive diplomacy, credibility and U.S. strategy. Those debates are worth revisiting now, as the Donald Trump administration confronts a Syrian civil war that is intensifying in dangerous ways.

The red-line saga began in August 2012. Responding to reports that Assad was preparing to use chemical weapons against rebels and civilians, President Barack Obama casually announced that such attacks would cross a U.S. “red line” and provoke an unspecified but presumably severe response. Assad nonetheless conducted escalating chemical attacks, culminating in a strike in the Damascus suburbs that killed more than 1,000 civilians in late August 2013.

True to its word, the Obama administration wound up for punitive military strikes in concert with the British and the French. Then everything went haywire. The U.K. dropped out after the government of David Cameron lost a parliamentary vote on whether to use force. Obama got cold feet, pulling back just as a strike seemed imminent and announcing that he would use force only if Congress authorized it. It quickly became clear that congressional authorization was not forthcoming — and that Obama himself was hesitant at best about striking Assad.

Yet as the administration headed toward an embarrassing retreat, the Russian government interceded by brokering a diplomatic settlement. That deal resulted in the removal of some 1,300 tons of Assad’s chemical weapons and averted U.S. military action.

Afterward, Obama’s defenders claimed that the episode was a testament to coercive diplomacy and the importance of exploring all options short of war. Critics warned that Obama’s climb-down would have disastrous effects for global perceptions of U.S. competence, credibility and power. Looking back, there was some truth in both arguments, but the costs of the decision now seem greater than the benefits.

In fairness, the Obama administration was in a tight spot in 2013, and the escape route it chose had advantages. As U.S. officials understood, even robust military action would not eliminate Assad’s chemical weapons. American ground forces could not strike the stockpiles themselves without the risk of releasing deadly toxins. And completely shattering Assad’s command-and-control structures carried some danger of allowing extremist groups to capture chemical-weapon components.

Yet the critics were also right in warning that Obama’s red-line diplomacy was fraught with drawbacks and dangers. We now know that Assad’s “disarmament” was far from complete, as the regime again launched major chemical weapons attacks on civilians in 2017 and 2018. In fact, the lesson Assad learned from the red-line episode seems to have been that he could intermittently employ chemical weapons while escaping meaningful punishment. Russian influence in Syria increased, presaging still-deeper involvement when the Kremlin intervened militarily to shore up Assad’s sagging regime in 2015.

For a global superpower with global security commitments, credibility is crucial. If U.S. credibility is robust, then friends and allies will feel secure, rivals and enemies will be kept in check, and the American-led international system will be fairly stable. If U.S. credibility crumbles, friends will feel insecure and enemies will be tempted to test American power. The long-term cost of the red-line episode was that it sent just the wrong message about American credibility at just the wrong time.

So does this mean Trump should hit Assad’s regime hard if it uses chemical weapons in its current offensive in Idlib province? The answer is less straightforward than it might seem, because the situation in Syria today is so much worse than it was in 2013.

Yet there is one clear lesson that Trump should heed: Draw your red lines carefully and enforce them vigorously. The Obama administration got in trouble for carelessly making a threat the president was not inclined to enforce. The Trump administration has made similar errors, by issuing a bewildering array of pronouncements about its aims in Syria, and by making threats — such as Trump’s promise to make Assad pay a “big price” for earlier chemical attacks — that it cannot or will not make good on.

Five years on, the red-line incident shows that credibility is a valuable commodity. U.S. presidents should not put it on the line unless they are willing to defend it aggressively when challenged.

Hal Brands is a Bloomberg Opinion columnist.