The northern border is still strictly guarded. King Felix was still under the tutelage of His Majesty Axel and the generals in turn. They still have to be vigilant because even though they have won the war, they haven't reached an agreement with King Felix despite suppressing him with various threats.
His Majesty Axel as the supreme leader still asked all of their soldiers to remain vigilant. I don't know what plan King Felix had, but the sudden attack on the northern border targeted the kingdom's territory and made His Majesty Axel more alert and full of projects. It is impossible to sit back and relax while wars and threats could come at any time.