A major witness in special counsel Robert Mueller’s investigation was arrested Monday on child-pornography charges, after flying into the U.S. for the first time since his cooperation with Mr. Mueller’s team. George Nader, a dual U.S. and Lebanese citizen, was charged in a criminal complaint more than a year ago, in April 2018, after he had spent hours with Mr. Mueller’s investigators detailing his contacts with Trump allies and efforts to set up meetings between Trump campaign associates and a Russian businessman close to President Vladimir Putin. Nobody connected to the Trump campaign was charged as a result of Mr....