A US Air Force F-22 Raptor stealth jet flies over a South Korean air base in Gwangju on December 4, 2017. The US and South Korea on December 4 kicked off their largest ever joint air exercise, an operation North Korea has labelled an "all-out provocation", days after Pyongyang fired its most powerful intercontinental ballistic missile. / AFP PHOTO / YONHAP / - / - South Korea OUT / NO ARCHIVES - RESTRICTED TO SUBSCRIPTION USE-/AFP/Getty Image