THE PENTAGON -- With two destroyers armed with cruise missiles positioned in the eastern Mediterranean, the U.S. appeared on the verge of launching a strike against the Syrian military in retaliation for the suspected chemical attack earlier this week.

Cruise missiles are unmanned aircraft which carry a 1,000-pound warhead. They fly close to the ground below enemy air defenses, guided to their targets by GPS satellites.

Preparations for the attack came in the middle of a high-stakes summit between President Trump and China’s President Xi at the Florida White House.

Mr. Trump is the second president to be on the brink of military action against the Assad regime.

“A red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized,” then-President Obama said in 2012.

And after a sarin gas attack in 2013 killed more than 1,400 Syrians, Mr. Obama prepared for airstrikes.

“The purpose of this strike would be to deter Assad from using chemical weapons, to degrade his regime’s ability to use them, and to make clear to the world that we will not tolerate their use,” he said.

But Mr. Obama backed down from his threat after Assad promised to hand over his stockpile of chemical weapons, a promise this week’s attack suggests the dictator did not keep.

That was nearly four years ago, before Russia had intervened in Syria on the side of the regime. If President Trump gives the order to strike this time, it will be carried out under the noses of the Russian military.