The Collaborative International Dictionary
Alien \Al"ien\, a. [OF. alien, L. alienus, fr. alius another; properly, therefore, belonging to another. See Else.]
Not belonging to the same country, land, or government, or to the citizens or subjects thereof; foreign; as, alien subjects, enemies, property, shores.
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Wholly different in nature; foreign; adverse; inconsistent (with); incongruous; -- followed by from or sometimes by to; as, principles alien from our religion.
An alien sound of melancholy.
--Wordsworth.Alien enemy (Law), one who owes allegiance to a government at war with ours.
--Abbott.
Usage examples of "alien enemy".
The alien enemy moaned en masse as limbs were severed and entrails ripped out.
That this defensive belt had ultimately fallen was due to nothing more than the sheer weight of numbers the alien enemy had thrown against it.
But she'd never poured a plascrete defensive wall or dug bomb shelters during a hurricane-force ice storm, under the guns of an alien enemy.
If she shows up, healthy and praising the alien enemy, then the war effort will be undermined.
It is not much of a hope, I admit, but it's the only logical thing they can do while they try and find the alien enemy first.