 If you liked
Full Metal Panic!
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 ...then you might like
Grisaia no Kajitsu
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Grisaia no Kajitsu is like what would happen if you put the protagonist of Full Metal Panic in the events of Bakemonogatari written by the guys of Key with help from Gen Urobuchi.
Both GnK and FMP are romantic comedy anime about teenaged secret military top soldiers sent to a normal school where they try to live "normal" lives. Both start out with lots of comedy, then become dark fairly quick. Although, while GnK stays mostly dark after, FMP goes back and forth with comedy.