The Russians are training the North Koreans to use stealthy,sophisticated drones. Soon they will be deployed against Ukraine and aquire not only expertise but combat experience. North Korean drone teams will be a serious threat to stationery targets such as airfields, military depots and important buildings throughout South Korea. Also the battered but still standing mullah regime in Iran will hire them to augment their surrogates. The drones are constantly innovating becoming more stealthy, more destuctive and very lethal.
During the day, drones could be optically spotted fairly simply looking upward except on clear blue-sky days. A contrail plane could produce a contrail against which an incoming drone can be spotted. Drones could also be optically spotted by protective spotting drones looking downward.
At night, drones might be detected when they block out an astronomical object.
An optically guided gun could then shoot down the drone.
In the daytime, a downward looking tracking rocket or drone might be launched ahead of the gunfire to track the incoming drone. An optical drone detection/tracking balloon is another possibility.
At night, a powerful rapidly scanning flashlight might illuminate an incoming drone.
Downward blast barrage balloons might also provide protection.