I *think* that in this case (since I suspect this is a translation) "aliens" could be taken as Auslanders/a military force from another country.
Reminds me of a cartoon I saw once. When I was a kid in January there would be ads on TV that all resident aliens had to report their address to the government. The cartoon shows a line of people doing this with a sprinkling of creatures from other planets.
(I looked for a smilie of a green space alien, but we don't have that one here.)