Astronomiensa neuvosta Aleksanteri Suuri päätti, ettei hyökkää Egyptiin, ja meni sen sijasta Intiaan.
I’m sure this sentence was originally intended to say “astrologers”, even though this version says astronomers in both languages. (Back in those days, military decisions followed everything from astrological readings to bird omens… but not so much real astronomy.)
I wonder whether “ettei hyökkäisi” wouldn’t be formally more correct.
It’d be better written as “…pääti olla hyökkäämättä Egyptiin”.
I don’t think the conditional (-isi) works here.
If I had to guess, I’d wager you’re probably going verbatim off of the English “decided that they would…”. But that’s an altogether different “would”—it’s the past tense of “will” (for something that, during the past timeframe of the narration, was still in the future), not a conditional form.