President Donald Trump on Thursday will visit a U.S. installation at the center of American involvement in the Middle East... Trump plans to address troops at Qatar’s al-Udeid Air Base, which was a major staging ground during the U.S. wars in Iraq and Afghanistan and supported the recent U.S. air campaign against Yemen’s Iran-backed Houthis. The president has held up Gulf nations like Saudi Arabia and Qatar as models for economic development in a region plagued by conflict as he works to entice Iran to come to terms with his administration on a deal to curb its nuclear program. The...