Find out where that invading army is planning to cross, erect a fence with concertina wire, and firehose anyone trying to cross it.
A powerful stream of water knocking a little girl off a fence is not something you want to see on the nightly news. So, there would need to be a media blackout. No reporters within 100 miles and a no fly zone of 100 miles. Shoot down all drones as soon as they enter the zone