They focused their efforts on pages that touched American nerves, with names like “Guns4Life,” “Pray for Police,” “Stop All Invaders,” “South United” and — mimicking Mr. Trump — “America First.”

Even pages that seemed nominally hostile to him often worked in his favor: “Woke Blacks” critiqued Mrs. Clinton for alleged hostility to African-Americans; “United Muslims of America” showed her with a woman in a head scarf and a slogan — “Support Hillary, Save American Muslims” — that seemed aimed at generating a backlash.

. . .

The Russian company’s formula was simple: tap into a simmering strain of opinion in the United States and pour on the fuel.

Consider the Texas protest. After the Russians put up the “Stop Islamization” Facebook post, several dozen like-minded Texans added their own incendiary comments. “Allah Sucks,” wrote one, adding a threat to kill any Muslim who tried to visit him. Another wrote of the Islamic center, “Need to Blow this place up.”

A dozen yelling white supremacists turned out for the protest, at least two of them with assault rifles and a third with a pistol. Others held Confederate flags and a “White Lives Matter” banner.

Houston police managed to keep them away from a much larger crowd of counterprotesters — some of whom had responded to a second Russian Facebook call. In a blatant attempt to create a confrontation, another Internet Research Agency page, this one called United Muslims of America, had asked people to rally at exactly the same time and place to “Save Islamic Knowledge.”