Even if the U.S. strike in Syria has not fundamentally changed the dynamic in the war, opening a new window of possibility, Washington has sent the powerful message that using chemical weapons carries a price.
President Trump's decision to launch nearly 60 Tomahawk cruise missiles against Al Shayrat air base, from which the Syrian air force flew to drop chemical weapons on the town of Khan Sheikhoun earlier this week, was swift and purposeful. No doubt, the horrific nature of the attack moved him. But the United States response was clearly about sending messages to President Bashar al-Assad and his allies, as well as the international community: Chemical weapons will not be used with impunity.
To be sure, this American strike, which was targeted and designed to inflict significant damage on one air base in Syria, will also convey to the Iranians, and to the North Koreans, that they had better take the words of this administration seriously. It is probably not without significance that it took place as the Chinese leader, Xi Jinping, North Korea's most important patron, was in Florida to meet with Mr. Trump.
While our adversaries will undoubtedly note that the United States' responses will not be limited to rhetoric when thresholds are crossed or American warnings are not heeded, our friends in the Middle East -- Arabs and Israelis alike -- will surely be heartened by this strike. Fairly or not, they had become convinced during the Obama administration that the United States was withdrawing from the region and its responsibilities there. They feared that the president saw Iran as part of the solution to the problems in the region and not a source of them. Those regional allies, too, will take the administration's words much more seriously, and may become more responsive to America's requests.
For friends and foes alike, then, this action will have an impact, particularly if it appears to be successful and affects Syrian, Iranian and Russian behavior. Time will soon tell whether Mr. Assad now chooses to test the United States by carrying out another chemical weapons attack. If he does so, he runs the risk of losing more of his air force and the major advantage it gives him over the rebels.
Of course, Mr. Assad could decide not to use chemical weapons and, instead, to increase his use of barrel bombs to try to terrorize the population in Idlib Province. He may reason that this won't draw a response and yet might allow him to take more territory in an area where the opposition forces remain.
That said, the regime's ground forces are stretched thin, and for them to take further territory depends on the Shia militias that Iran has brought into Syria from as far away as Afghanistan. Are the Iranians ready to up the stakes in Syria in response to this American action?
They have invested a great deal in preserving Mr. Assad in power, but are they ready to commit more? All the more so when Mr. Assad was the one whose use of chemical weapons triggered this change in American policy -- within days of Secretary of State Rex W. Tillerson's saying that Mr. Assad's future "will be decided by the Syrian people." It is one thing to keep Mr. Assad in power, another to try to take back, in Mr. Assad's words, "every inch."
Iran has additional options if it wants the United States to pay a price for carrying out this strike. It could use its Shia militia proxies to attack American forces either in Syria or in Iraq. But before doing so, Iran's leaders are likely to think about whether they really want to undermine or weaken the American effort against the Islamic State, an enemy that directly threatens Iranians and Shiite Iraqis.
And what about the Russians? Could they deploy more forces to Syria to raise the costs of any escalation of American action, or could they decide it is time to make clear to Mr. Assad that they will no longer provide him with protection? The initial Russian response of condemning the strike, claiming that the chemical weapons were the rebels', not the Syrian government's, and suspending the deconfliction arrangement, appears to double down on their bet with Mr. Assad. But it may be more a case of President Vladimir V. Putin not wanting the American use of force to appear decisive.
Mr. Putin has achieved much of what he wants in Syria: securing the regime, acquiring an air base, expanding a naval facility and being an arbiter of any outcome. This is a time to look for a way to consolidate these gains, not to raise the costs of Russian involvement.
It is too soon to know whether any of these actors will test the administration. But the president and his administration should not be passive and wait to see what happens next. They should be conveying privately to the Russians, Iranians and Syrians not to test us, not to play with fire. With Russia, in particular, the message should be: The insurgency against Mr. Assad is not going away, so if you don't want to be stuck in Syria at a time when the price may go up, we are willing to work with you to implement the principles embodied in the Geneva peace process.
Diplomacy often needs to be backed by a coercive element, and the military strike may give the Russians the incentive they have lacked to implement the principles they backed in United Nations Security Council Resolutions 2254 and 2268: a cessation of hostilities, an end to sieges, unencumbered access for humanitarian assistance and an 18-month period for political transition.
It is possible that the American strike has changed the dynamic in Syria and created a new possibility. Unfortunately, in a conflict that has produced a humanitarian catastrophe, it could also be just one more step in a war that may not end until all sides are exhausted.
In spite of that, the United States has sent a powerful message that there is a price for using chemical weapons. That message needed to be sent.
Dennis Ross is the counselor and William Davidson Distinguished Fellow at The Washington Institute.