In late January, Mansoor Shams, a 34-year-old former Marine corporal, was running through Seattle-Tacoma National Airport when a woman stopped him. She wanted to talk about the whiteboard sign he was carrying. “I’m a Muslim and a U.S. Marine,” it read in thick red and blue marker. “Ask anything.” He had considered erasing the message. As a Muslim, he’d never felt especially comfortable in airports. But the woman had a big smile on her face. She just wanted to give Shams a high five.