The United States launched more than 50 tomahawk missiles Thursday night at al-Shayrat military airfield, a base used by both Syrian and Russian military forces in Syria’s ongoing civil war.

The airfield isn’t just a valuable military target: It was from al-Shayrat that the Syrian government launched its chemical-weapons attack Tuesday on Idlib province’s Khan Sheikhun, a rebel-held city.

After the U.S. missile launch Thursday night, President Donald Trump himself confirmed that the air base, near Homs, was the origin of the chemical-weapons strike, a fact that some news outlets had reported earlier in the day. Just before the president addressed the nation, an official told Voice of America there was a “high degree of confidence” the attack came from the airfield, “under the command” of Syrian President Bashar al-Assad. Earlier Thursday, BuzzFeed News reported that a “network of local monitors” inside the country tracked Syrian jets departing al-Shayrat on Tuesday, which were later seen “circling Khan Sheikhun before the attack.”

How the Trump administration would respond to the deadly strike on Khan Sheikhun seemed murky just hours before Thursday’s missile launch. During a press conference Wednesday, Trump hinted at the potential for a military response, but went no further. By Thursday afternoon, CNN reported that he had clued in members of Congress that he was “considering military action,” and Reuters alleged strategy talks between the Pentagon and the White House were already under way.

The American missiles targeted Syrian fighter jets “and other infrastructure” at the base, according to a senior military official who spoke to The New York Times. The Washington Post reports that the missiles zeroed in on “air defenses, aircraft, hangars, and fuel.”

But Russian assets, stationed at al-Shayrat since at least late 2015, were reportedly avoided in the strike. In December of 2015, just months after Russia first intervened in Syria, Agence France-Press reported that Russia was “reinforcing” the base and building additional runways before beefing up its operations there. The Syrian Observatory for Human Rights, which monitors the ongoing war in Syria, reported over a year later that Russia was still expanding the base, which was to be “used by Russian warplanes in operations around the [ISIS]-held city of Palmyra.” And just last month, Russia sent a handful of its new attack helicopters there.

On Thursday, U.S. officials gave the Russian government a heads-up that the attack was coming, the Times reports:

[A U.S. official] said that no Russian planes were at the airfield and that the cruise missiles did not target any Russian facilities.

The Pentagon informed Russian military officials, through its established deconfliction channel, of the strike before the launching of the missiles, the official said, with American officials knowing when they did that that Russian authorities may well have alerted the Assad regime. “With a lot of Tomahawks flying, we didn’t want to hit any Russian planes,” he said.

In a statement, Pentagon spokesman Captain Jeff Davis said the United States “took extraordinary measures to avoid civilian casualties,” and “every precaution was taken to execute this strike with minimal risk to personnel at the airfield.”

It’s not clear how the Syrian government will react to this latest assault. But for now, it seems the United States has avoided sparking immediate backlash from Russia, with whom it maintains an uneasy relationship in the area. As my colleague David Graham wrote earlier Thursday: “If American attacks killed Russians, it could spark a much larger diplomatic or military encounter.”