Two Defense Department officials said the Syrian military had moved some of its key aircraft to a Russian base, assuming the Americans would be reluctant to strike there. Russian commanders have also moved some of their military forces in anticipation of American action.
“You want to hit military targets, military equipment as much as possible, because it’s the Syrian military that’s carrying out these atrocities,” said Andrew J. Tabler, a Syria scholar at the Washington Institute for Near East Policy. “You want to make sure that you deliver a message and that you degrade their military capabilities.”
At the same time, he added, “the risk is, there are a lot of Russians throughout Syria.”
“They’re claiming they have people at every Syrian base,” he continued. “If you end up killing Russians, that risks a confrontation with Russia.”
Mr. Trump said earlier this week that he would respond to Saturday’s suspected chemical attack within 24 to 48 hours. But the move toward military action has slowed as the administration sought to coordinate with allies, including France and Britain.
A joint operation takes longer to organize but would avoid the United States looking as though it was acting on its own.
Prime Minister Theresa May of Britain called a cabinet meeting for Thursday, and the BBC reported that she was ready to join a military operation without seeking approval from Parliament, as her predecessor, David Cameron, did in similar circumstances in 2013 only to be rebuffed by lawmakers. Mrs. May ordered British submarines to move within missile range of Syria, according to The Daily Telegraph.
President Emmanuel Macron of France, who is scheduled to visit Washington this month for a state dinner at the White House, has made clear that he is determined to participate in a strike as well. France has warplanes armed with cruise missiles in nearby Jordan and the United Arab Emirates.
Mr. Trump left little doubt about his intention with an early-morning Twitter post on Wednesday.
“Russia vows to shoot down any and all missiles fired at Syria,” he wrote. “Get ready Russia, because they will be coming, nice and new and ‘smart!’ You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!”
The message conflicted with Mr. Trump’s oft-stated scorn for President Barack Obama for, in his view, forecasting his military moves. “No, dopey, I would not go into Syria,” Mr. Trump wrote on Twitter in 2013 when Mr. Obama was considering a strike of his own in retaliation for a chemical attack on civilians, “but if I did it would be by surprise and not blurted all over the media like fools.”
Sarah Huckabee Sanders, the White House press secretary, said on Wednesday that the president was not violating his own policy because he did not give a precise time for the attack to begin.
“The president has not laid out a timetable and is still leaving a number of other options on the table,” she said. “And we’re still considering a number of those, and a final decision on that front hasn’t been made.”
Defense Secretary Jim Mattis said that the United States was still evaluating the intelligence on the suspected chemical attack on Saturday that killed dozens in Douma, a suburb of Damascus. “We’re still assessing the intelligence, ourselves and our allies,” he told reporters. “We stand ready to provide military options if they’re appropriate, as the president determined.”
Few, if any, doubt the American capacity to inflict damage on Mr. Assad’s government. But it remains unclear whether the operation envisioned by Mr. Trump will be any more meaningful than a cruise missile strike he ordered last year after a chemical attack. That strike hit a Syrian air base that was up and running again within 24 hours.
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“The question then becomes, are we just going to try to add additional costs on Assad and see and hope that it establishes a more effective deterrence?” said Jennifer Cafarella, a senior analyst at the Institute for the Study of War. “Or is President Trump going to, no kidding, pursue an effective deterrence that holds not just Assad, but his external backers, accountable as well?”
Military operations can produce unintended consequences and diplomatic nightmares.
President Bill Clinton’s airstrikes against Al Qaeda targets in 1998 missed Osama bin Laden in Afghanistan and hit a pharmaceutical plant in Sudan that turned out not to be the chemical weapons facility American intelligence analysts thought it was. Mr. Clinton’s air campaign to protect Kosovo from Serbian forces a year later resulted in the accidental bombing of the Chinese Embassy in Belgrade.
Accuracy of American weaponry has improved since then, but mistakes and limitations remain facts of war.
In February, a clash in Syria between pro-government forces backed by Russian mercenaries and a largely Kurdish militia that is supported by the United States left an undetermined number of the Russians dead.
Just days later, Syria demonstrated that its air defenses could threaten foreign warplanes when an Israeli F-16 fighter jet crashed after coming under heavy fire, the first Israeli plane lost under enemy attack in decades.
In the last three years, the Syrian military has significantly upgraded its air defense systems, mostly with help from Russia, a former senior Defense Department official said. Although surface-to-air missiles would likely threaten American aircraft in western Syria, those jets would be able to fire cruise missiles from hundreds of miles away, either out at sea or over a neighboring country.
While Syrian air defenses have the range to hit an American jet flying above a country such as Lebanon, the American military could recover the downed aircrew much more easily, the former official said.
With American intentions so clearly forecast by Mr. Trump, the Syrian government has moved aircraft to the Russian base near Latakia, and is taking pains to secure important weapons systems, military analysts said. Russia, too, has had several days now to move key personnel and equipment out of harm’s way.
The United States and Russia still maintain a so-called deconfliction channel between American forces at Al-Udeid Air Base outside Doha, Qatar, and Russian officials at the Hmeymim military base in Syria, a Defense Department official said on Wednesday. The Pentagon alerted Russia before last year’s strike to warn its personnel to steer clear.
If Russians or Iranians were killed in a strike, it is unclear how their countries would react. The former senior Defense Department official expressed less concern about American attacks on the Russians because of the deconfliction line set up but sounded wary about a missile striking Iranian troops or their proxies.
The Iranians, the official said, could easily escalate militarily, by attacking American troops along the Euphrates River in Syria or with Iranian-backed militias in Iraq.
Pentagon officials said that even if Syrian warplanes eluded an American-led strike campaign, the United States and its allies could still damage airfields across the country to hamper Mr. Assad’s ability to launch future chemical weapons attacks. That kind of damage, though, would require a sustained strike campaign, likely over several days.
Derek Chollet, a senior Pentagon official during Mr. Obama’s deliberations in 2013, said the United States was in a better position now than it was then. After that episode, Mr. Obama dispatched American forces to Syria to fight the Islamic State terrorist group and that experience, Mr. Chollet said, has benefited the United States.
“We have a much better sense of the threat picture, about Syrian antiaircraft defenses and targeting because for four years now we have had people picking targets in Syria,” he said. “Now, those were ISIS targets, but our knowledge of the terrain is so much greater.”