WASHINGTON — U.S. President Donald Trump's decision to launch Tomahawk missiles at a Syrian air base was the culmination of 48 hours of activity, one that began with what the U.S. believes was a chemical warfare attack on civilians perpetrated by the regime of Syrian President Bashar Assad.

Two senior defense officials briefed reporters Friday at the Pentagon about details of the strikes, in the process laying out their best understanding of the timeline from the April 4 chemical weapons attack to the decision last night to launch high-end weapons at Shyrat airfield.

April 4

6:50 a.m. local time — A Syrian fixed-wing aircraft, most likely a Su-22 launched from Shyrat airfield, drops a munition that strikes in the middle of a street at Kahn Sheikoum. "We know the routes that these aircraft took. We know that these aircraft were overhead at the time of the attack," the first Pentagon official said.

Subsequent analysis of the crater caused by the missile would show "staining" around the explosion site, in line with the signs of a chemical warhead. Despite early claims from Russia and Syria that the chemicals came from an explosion at a chemical weapons factory being maintained by anti-Assad rebel groups, the Pentagon official said the crater evidence shoots that option down.

7 a.m. local time — Intelligence shows the first "reflections" of the potential use of a nerve agent, which include causalities arriving at a local hospital. Shortly afterward, a small unmanned system —  "either regime or Russian" — flies over the hospital, gathering intelligence at the scene before departing. "About five hours" after it was first spotted, the UAV returns, and shortly thereafter the hospital is struck by a fixed-wing aircraft.

"We don't have positive accountability yet, but [why] somebody would strike the hospital, potentially to hide the evidence of a chemical attack about five hours after it was clearly seen that was a hospital with ambulances and civilian traffic, is a question that we're very interested in," the first official said.

April 5

Trump directs Secretary of Defense Jim Mattis to come up with military options to respond to the chemical attacks.

According to the second defense official, "those options were basically put together into recommendations. It went forward to the National Security Council, multiple meetings with not only the presidential senior advisers but also the chairman [of the Joint Chiefs], the vice chairman and the Secretary of Defense."

April 6

Final proposals for action are presented to the president, with the final selection made sometime in the afternoon.

4:30 p.m. EST — The Pentagon receives orders from Trump to execute the plan.

7:40 p.m. EST — U.S. Navy guided-missile destroyers Porter and Ross combine to launch 60 Tomahawk cruise missiles at Shyrat air base. One of those weapons fails to launch, requiring the Navy to launch a replacement; while heading to the target, one of the missiles fails and ditches into the water.

8:40 p.m. EST — Fifty-nine Tomahawk weapons reach their targets. Each missile was assigned to a single target and detonated successfully.

What was targeted

  • Syrian military assets: Fuel, hardened bunkers, munitions and a Russian-made surface-to-air missile system were targeted by the strikes.
  • Syrian planes: The Pentagon believes about 20 Syrian aircraft were destroyed, though it is hard to say exactly how many were taken out, as some planes were inside bunkers that were destroyed.

Asked to assess how losing those planes would degrade Assad’s military, the official declined to offer a figure but noted that "20 aircraft out of their inventory is going to make an impact."

What wasn't targeted

  • Chemical warfare bunkers: The Pentagon was careful to avoid hitting anything they believed to be storage for chemical warfare materials, in order to not unleash those weapons.
  • The runway: Shyrat has a 10,000-foot runway, but that wasn’t targeted in the strike. Asked why, the officials said that didn’t fit into the "proportionality" they were going for and noted that, as they were using Tomahawks, "it would have been a waste of a munition on the airfield."
  • Russia: The Pentagon believes there are up to 100 Russian personnel on the base, and the mission plan was created specifically so the strikes would not target Russian citizens or assets. Ahead of the strikes, Russia was warned the Tomahawks were incoming so they would not "read the attack" wrong, the second official said.

During the briefing, the officials were careful not to formally link Russia to the chemical weapons attack. But in both tone and their statements, the officials acknowledged that if there was an active chemical weapons storage unit at the air base, the Russians would likely have known about it. They also would not rule out that the Russians played a part in the hospital bombing.

"We know the Russians have chemical expertise in country. We cannot talk openly about any complicity between the Russian and the Syrian regime in this case, but we’re carefully assessing any information that would implicate the Russians knew or assisted in this Syrian capability," the first official said.

"We have no knowledge of Russian involvement in this attack. But we will investigate any information that might lead us in that direction. We’re not done."