WASHINGTON — President Trump ordered a cruise missile strike against Syria early Friday local time in retaliation for the chemical weapons attack that killed 86 people on Tuesday, according to media reports.

The attack, the first conventional assault on another country ordered by Trump, comes a day after he declared that the chemical weapons assault had “crossed many, many lines,” including the deaths of 27 children.

The 50 or so missiles, fired from a U.S. Navy vessel in the Mediterranean Sea, struck the airfield where the Syria based the warplanes used in the chemical attack.

The attack essentially follows the plan that the Pentagon had set in September 2013, according to a senior Defense official not authorized to speak publicly about the operation. That plan was devised after President Obama had set a “red line” on the use of chemical weapons. Syrian President Bashar Assad had used the weapons that killed 1,400 civilians, but Obama did not order an attack. Instead, Assad agreed to turnover his stockpiles of chemical weapons, a pledge he obviously reneged on in light of Tuesday’s use of what experts believe was sarin gas on civilians.

In 2013, military planners had planned to use land-attack cruise missiles launched from Navy destroyers cruising off shore from Syria. For weeks, the Navy had four destroyers floating off shore, waiting for the order to strike that never came.

Using ships negates the need to seek permission from countries where U.S. warplanes are based. Land-attack Tomahawk missiles can travel 1,500 miles to strike their target and carry a warhead with 1,000 pounds of conventional explosives.

Read more:

► Analysis: Trump gets his blame game on, targets Obama for just about everything

► 'Steps are underway' to oust Syria's Assad, Tillerson says

► Trump team weighs military response in Syria and ways to remove Assad

Among the options that Pentagon planners had developed for Trump: the airfield, military command-and-control centers, air defense systems and troops.

Any attack puts at risk the hundreds of U.S. special operations troops in eastern Syria who are advising local ground forces in their fight against the Islamic State, or ISIS. The concern, according to the official, is that Assad could order a counter strike, targeting the Americans. There is also the risk that the attack could kill Russian troops who have been supporting the Assad regime.

The Pentagon, which has been bombing ISIS targets in Syria since 2014, can provide extra air patrols to protect those troops. But they still would be vulnerable to attacks by surface-to-surface missiles fired by Syrian forces.