President Trump extended both support and a suggestion in the hours following the terrorist attack in Barcelona.

Barcelona, Spain, on Thursday became the site of the most recent attack involving a car driven into a crowd, with at least 13 reported dead, notes The Guardian.
President Trump extended both support and an apparent suggestion in the hours following the incident. 
He first wrote, “The United States condemns the terror attack in Barcelona, Spain, and will do whatever is necessary to help. Be tough & strong, we love you!” 
About 45 minutes later, he tweeted, “Study what General Pershing of the United States did to terrorists when caught. There was no more Radical Islamic Terror for 35 years!”
This is not the first time Trump has referenced Pershing. 
At an early 2016 rally held in North Charleston, South Carolina, Trump told a story. 
According to the Washington Post, Trump noted that in the Philippines more than a century ago, Pershing "...took the 50 terrorists, and he took 50 men and he dipped 50 bullets in pigs’ blood...He took 50 bullets, and he dipped them in pigs’ blood. And he had his men load his rifles, and he lined up the 50 people, and they shot 49 of those people. And the 50th person, he said: 'You go back to your people, and you tell them what happened.' And for 25 years, there wasn’t a problem."
Politifact, a fact-checking outlet, wrote, “Of the eight historians we checked with, all were at least skeptical that what Trump said actually happened…Perhaps more important, the historians took issue with Trump’s suggestion that the tactic -- if it was even used at all -- actually worked to end tensions, noting that unrest persisted for years.”