SEOUL, South Korea — South Korea and the United States will launch their annual large-scale military exercise this month to bolster readiness against North Korean threats, the allies said Thursday, in a move likely to irritate Pyongyang amid a prolonged stalemate in diplomacy. The exercise also comes against the backdrop of concerns in Seoul that the Trump administration could shake up the decades-old alliance by demanding higher payments for the U.S. troop presence in South Korea and possibly