A U.S. Air Force F-16 fighter takes off during annual exercises with South Korea at a U.S. air base in Gunsan, South Korea, in April. North Korea implied the summit with President Trump could be canceled because of the exercises.

A U.S. Air Force F-16 fighter takes off during annual exercises with South Korea at a U.S. air base in Gunsan, South Korea, in April. North Korea implied the summit with President Trump could be canceled because of the exercises.