Translation:The soldiers had discovered the enemies.
'the soldiers had discovered the enemy' should be allowed since 'enemy' and 'enemies' in English are interchangeable.
Except that "The soldiers had discovered enemies" isn't accepted for me, five months later, even though it's much more natural-sounding English. (I hope you know I don't post criticisms to be nasty. I think we all -- well, most of us, I've seen an exception or two -- are thankful for Duolingo and want it to be all it can be.)
I don't mind at all - quite the contrary: user input is the best way of improving the course.
The Swedish fienderna means "the enemies" in the definite though. I agree that the indefinite is more natural in English in general, but it doesn't quite mean the same thing, and you could very well just say fiender in Swedish as well. (Of course, there's also fienden - "the enemy" which might be the best of the three in general.)
Thanks once again for your graciousness, not to mention hard work. I realize you are basically trying to teach two (and more) languages at the same time -- something I never had to cope with back in my teaching days, plus techie stuff I wouldn't have the faintest notion of how to deal with except to go berserk, which doesn't seem exactly the optimal solution either. Having mulled it over for a bit and the current geopolitical situation of Sweden and Germany, maybe "fienderna" is the best solution of all? :-(