Simplify the following text for a Grade 9 native English-speaking person.
Original: On July 25 , 1944 , the German occupation forces were engaged in battle by fighters of the Polish underground Armia Krajowa , joined by Polish partisans of the Bataliony ChÅ opskie .
Simplified: On July 25 , 1944 , the German occupation forces were attacked by fighters of the Polish resistance .