 Complete the following phrase:  Although not under von Bocks command, Kesselrings purpose was to support Army Group North in closing the Polish Corridor from the third day, with emphasis thereafter on supporting the 3rd Army as it advanced along the Vistula to isolate Warsaw from the east. From 8 September Kesselring began interdiction operations against bridges as German forces advanced on Warsaw. When a powerful Polish counter-attack created a crisis, he contributed Fliegerdivision 1 to the 