Simplify the following text for a Grade 9 native English-speaking person.
Original: In the first , called Fall Gelb in German ( called Case Yellow in English ) , German tank units pushed through the Ardennes , to circle the Allied units that moved into Belgium .
Simplified: In the first , Fall Gelb ( Case Yellow ) , German armored units pushed through the Ardennes , to cut off and surround the Allied units that had advanced into Belgium .