WASHINGTON – The Pentagon on Friday laid out its case for why it believes the Syrian regime conducted the April 4 chemical weapons attack, which led to President Trump ordering a cruise missile strike against a Syrian military airbase.

The strike was the first time the U.S. has purposely targeted the Syrian government since the 2011 civil war broke out there.

The strike has major implications for the U.S.’s involvement in the war, which President Obama sought to avoid. It raises the question of what else Trump may do, particularly if the Syrian regime continues to target civilians and opposition rebels. It has also angered Russia, whose government Trump has said he wanted to improve relations with.

Senior U.S. military officials are suggesting that Russia may have played a role in the chemical weapons attack which, if proven, would raise the question of what the U.S. would do in response.

The potential for more war in Syria is troubling for Trump’s political base, many of whom believe that the U.S. should not get entangled in foreign wars.

Two senior U.S. military officials speaking to reporters on background at the Pentagon laid out the justification for the strike against the Syrian regime, which saw two U.S. destroyers launch 59 Tomahawk missiles at a military airbase in Al-Shayrat in western Syria.

They said they believe the Syrian regime conducted the April 4 chemical weapons attack in Khan Sheikhoun, a small city in Idlib province, because they were at risk of losing an important airfield to the Syrian opposition in neighboring Hama province.

“We think that this attack is linked to a battlefield desperation decision to stop the opposition from seizing those key regime elements,” one official said.

The military officials pointed to two recent alleged chemical weapons attacks in Hama, which involved chlorine and an unidentified nerve agent on March 25 and March 30.

“This escalated on 4 April to use of chemical weapons material,” the official said.

Officials also said they tracked a Syrian regime aircraft, likely an SU-22, flying from the Shayrat airbase to Khan Sheikhoun on the day and time of the attack, showing reporters a graphic of the tracked flight.

Sr mil official: On day of the CW attack, at the time of the attack, we have these aircraft tracks. They go fm al Shayrat 2 Khan Shaykhun. pic.twitter.com/yiqqv6MKdu — Kristina Wong (@kristina_wong) April 7, 2017

“We know the routes these aircraft took. We know that these aircraft were overhead at the time of the attack,” one official said.

They disputed Russian reports that the munition struck a rebel chemical weapons facility, showing pictures of a crater in the middle of a street where they say the chemical weapon landed.

They said the crater showed staining around it that was a telltale sign of a chemical munition.

They also said munition fragments suggest it was a type of weapon not aimed to create heavy damage, another sign it was meant to distribute chemicals.

Sr US mil official disputes Russian report that opposition chemical weapons factory was struck.” Shows pic of where munition hit, in street. pic.twitter.com/NTRbj3HBSr — Kristina Wong (@kristina_wong) April 7, 2017

“We highly suspect, and in fact, there’s no credible alternative to a Syrian regime air attack as the source of the chemicals that killed so many Syrian civilians,” the official said.

“There’s no credible opposition capability with nerve agents,” they added. “ISIS has experimented with mustard agents, blister agents, not nerve agents. But this is well beyond the technical capability of the opposition.”

The officials said they “suspect” the Syrian regime had help — pointing the finger at the Russians, who they say share the base with the Syrians and “obviously” would have known every flight operation that comes out of the airfield.

At the time of the strikes, there were anywhere between 12 to 100 Russian personnel at the base, including a helicopter squadron and support crew.

“We think we have a good picture of who supported them as well. Obviously, at a minimum, the Russians failed to rein in the Syrian regime activity. And, again, the continued killing of innocent Syrian civilians,” the official said. “We know the Russians have chemical expertise in the country. We cannot talk about — openly — any complicity between the Russians and the Syrian regime in this case. But we’re carefully assessing any information that would implicate the Russians knew or assisted with the Syrian capability.”

However, it remains unclear whether the Russians had a direct role in the chemical weapons attack.

One official said that, shortly after it was launched, a small drone of unknown nationality flew over the hospital where patients were being taken to. It left and returned five hours later, when a fixed-wing aircraft — also of unknown nationality — struck the hospital.

“We don’t know why somebody — who struck that. We don’t have positive accountability yet, but the fact that somebody would strike the hospital, potentially to hide the evidence of a chemical attack, about five hours after — it was clearly seen that it was a hospital with ambulances and civilian traffic — is a question that we’re very interested in,” the official said.

But officials clarified that they had “no knowledge of Russian involvement in the attack.”

“But we will investigate any information that might lead us in that direction,” one official said.

The senior U.S. military officials said they went out of their way to not hit Russian facilities, troops, or equipment at the base, and that they knew where they were located on the base and took care not to strike in those areas.

Officials said they hit an estimated twenty aircraft, hardened aircraft shelters, petroleum and fuel storage sights, and a surface-to-air missile system — targets that will make the airbase inoperable. Targeting the runways, which are easy to fix, would not have been worth the missile, they said.

The officials said the decision to strike came down from Trump at 4:30 p.m. on Thursday — just four hours before missiles began to fly.

The Russians were notified once of the oncoming strike, they said, through a military channel they use to make sure there are no accidents between their forces in Syria.

The officials said it was “implied” that the Russians will tell the Syrians at the base, and that it was “quite possible” aircraft could have slipped out, but that they did not see any leave, nor did they see any response from Russian air defenses.

The officials said there were “multiple” military options the President was given to choose from, and the strike was the “proportional response” to the chemical weapons attack.

The strike could possibly save “hundreds of civilians, probably thousands” from future chemical weapons attacks, one official said.

“We had to create a deterrent for that behavior,” the official said.

The military is conducting a thorough battle damage assessment to account for all the damage, which would likely include any civilian casualties. Syrian government media have claimed there were nine casualties, including four children, but the U.S. military officials say they have no evidence of that yet.

So far, Russia has also reacted with anger, declaring that it will cut a channel between U.S. and Russian forces in Syria meant to make sure accidents between them do not happen.