Secretary of State Rex Tillerson joined almost one million bikers on Sunday as a rider in a massive caravan of motorcycles that rode from the Pentagon to the National Mall and the Vietnam Memorial on Sunday. A video posted on YouTube shows Tillerson posing for pictures with other bikers and preparing to mount his own bike for the trip, an annual tradition that began in the late 1980s as a way to honor the U.S. military and to support the continuing search for U.S. soldiers who remained prisoners of war or were missing in action following the Vietnam War. “O.K.,...