"The king won the battle."
Translation:Ο βασιλιάς νίκησε στη μάχη.
A google serarch on "κερδισε τη μαχη" gets about 48000 hits, but "νικησε τη μαχη" only about 3000, supporting HellasCad. I tried the first one as my first answer, on the basis of my limited but not completely zero knowledge of Greek, and it was rejected, which shouldn't have happened.
The word "στη" is the abbreviated form of the words "εις" and "την or τη" (depended the grammar fall of the following word), which used together in the spoken Greek language of the previous century. In today's spoken language we say simply "σ'τη" or "στην".
In any case it means "In the".