President Donald Trump’s eldest son was likely paid at least $50,000 for an appearance late last year before a French think tank whose founder and his wife are allies of the Russian government in efforts to end the war in Syria.

Donald Trump Jr. addressed a dinner on Oct. 11 at the Ritz Hotel in Paris, hosted by the Center of Political and Foreign Affairs. Its president, Fabien Baussart, and his Syrian-born wife, Randa Kassis, have cooperated with Russia in its drive to end the Syrian civil war, according to U.S., European...