WASHINGTON, D.C. — Turkey has agreed to let the U.S. military launch airstrikes against the Islamic State from a key air base near the Syrian border, senior U.S. officials said Thursday, giving a boost to the U.S.-led coalition while drawing Turkey deeper into the conflict. President Barack Obama and Turkish President Recep Tayyip Erdogan finalized the deal in a phone call Wednesday, officials said, following months of U.S.