The U.S. launched cruise missiles against Syria on Friday, a day after President Trump said a chemical weapons attack that killed at least 86, including dozens of women and children, “crossed many, many” lines.

Here’s what we know so far:

What's the background to the attack?

In 2013 President Obama set a "red line" against the use of chemical weapons by Syria's President Bashar Assad. The regime proceeded to use the weapons to kill about 1,400 civilians, including hundreds of children, according to United States government estimates at the time, but Obama decided not to attack — a move Trump and other Republicans widely criticized as making America look weak.

In a Russian-brokered deal in the wake of that episode, Assad agreed to turn over his stockpile of chemical weapons. This week's chemical attack clearly violated that pledge. Trump called the attack "a disgrace to humanity" and "truly one of the egregious crimes."

"The strike was intended to deter the regime from using chemical weapons again," Navy Capt. Jeff Davis, a Pentagon spokesman, said in a statement.

How did the U.S. carry out the strike?

Fifty-nine Tomahawk cruise missiles were fired from the destroyers USS Porter and Ross in the eastern Mediterranean Sea, targeted the Shayrat Airfield where Syria based the warplanes used in the chemical attack, according to Navy Capt. Jeff Davis, a Pentagon spokesman. The missiles destroyed aircraft, hardened hangars, ammunition supply bunkers, air defense systems and radar at the base.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said “the combat efficiency of the U.S. strike was very low" and that only 23 of the 59 missiles fired by the U.S. reached the base. "The place of the fall of the other missiles is unknown," Konashenkov said, according to Russia's TASS news agency.

The plan for the attack followed one devised in 2013 after Obama set his "red line," a senior defense official told USA TODAY.

Did the strikes cause any casualties?

The official SANA news agency said nine civilians, including four children, were killed in the strike. Russian spokesman Igor Konashenkov said six Syrian jets were destroyed but the air base's runway was intact.

Earlier, the Syrian army said the strikes killed six people, SANA reported, though it was not clear if that included the civilian casualties. Barazi told The Associated Press that the U.S. missile strikes killed three soldiers and two civilians and wounded seven other people. A Syrian opposition monitor said the attack killed four soldiers, the AP reported.

Has the U.S. struck Syria before?

The U.S. has been bombing Islamic State targets in Syria since 2014, but this was the first strike against the Syrian regime. This also marked the first conventional assault on another country ordered by Trump.

Why did the U.S. attack from ships?

Tomahawk missiles can travel as many as 1,550 miles to strike their target, so the U.S. Navy was able to launch the attack from the Mediterranean, avoiding the need to get permission from a host country to use aircraft or land-based missiles.

What Trump and aides said about the decision:

"It is in the vital national security interest of the United states to prevent and deter the use of deadly chemical weapons," Trump said from his Mar-a-Lago retreat Thursday evening after the strikes were carried out. "Assad choked out the lives of helpless men, women and children. It was a slow and brutal death for so many. Even beautiful babies were cruelly murdered at this very barbaric attack. No child of God should ever suffer such horror."

National Security Adviser H.R. McMaster said Trump was highly engaged in the Syrian attack after U.S. intelligence agencies quickly identified the source of the attack and the chemical agent used. “That confidence level has just continued to grow in the hours and days since the attack,” McMaster said.

Secretary of State Rex Tillerson said high-level Russian officials were not consulted ahead of the attacks. "There were no discussions or prior contacts – nor have there been any since the attack – with Moscow,” he said late Thursday. But U.S. forces did implement the so-called “deconfliction” agreement with Russian forces in the region in an effort to minimize Russian casualties. Russia later suspended the agreement, according to CNN.

What are the risks of attacking the regime?

One potential concern is the safety of U.S. special operations troops in eastern Syria who are advising local ground forces in their fight against the Islamic State (ISIS). One official told USA TODAY there are fears Assad could counter by targeting the U.S. troops.

One reason the U.S. chose limited strikes against Assad is that a more sustained campaign would risk the total collapse of the regime. That risks a power vacuum that could allow ISIS or other extremists to seize power.

How did Syria respond?

Syria condemned strikes as a "blatant act of aggression." The Syrian army said the action made the United States a “partner” of ISIS, Jabhat al-Nusra (an offshoot of al-Qaeda) and other terror organizations.

The office of Syrian President Bashar Assad said the strikes were "shortsighted," “reckless” and “irresponsible.”

“At 3:42 a.m. today, the United States of America committed a blatant act of aggression targeting one of our air bases in the Central Region with a number of missiles, leaving 6 people martyred and a number of others injured and causing huge material damage,” the Syrian military said in a statement carried by the official Syrian Arab News Agency.

How did Russia and Iran respond?

"It is an act of aggression under a completely far-fetched pretext," Russian Foreign Minister Sergey Lavrov said Friday, according to the government-owned Russian news agency TASS. "This is reminiscent of the situation in 2003, when the U.S. and the U.K., along with some of their allies, invaded Iraq without the consent of the U.N. Security Council and in violation of international law," he said.

Iran said the U.S. action was "dangerous, destructive and violates the principles of international law.” Iran’s Foreign Ministry spokesman, Bahram Ghasemi, said Iran condemned the strikes “regardless of the perpetrators and the victims” of the chemical weapons attack in comments carried by the semi-official ISNA news agency.

One concern was how Russia and Iran would react since both countries have boots on the ground in Syria supporting the Assad regime. Iranian militias support Assad and Russia's intervention turned the tide of the six-year-old civil war in Assad's favor.