In a visit to the heavily armed border with the South, the North Korean leader, Kim Jong-un, ordered his troops on higher alert, escalating his militaristic language in spite of American calls to improve ties with the Seoul just a week after his country agreed to a nuclear freeze in return for badly needed food aid. .Mr. Kim has been hailed as the North’s leader since the death of his father, Kim Jong-il, in December, and has frequently visited frontline military units. But his trip to Panmunjon, a compound straddling the border where the armistice ending the Korean War was...