The geography of both New York and Philadelphia played a major role in establishing the two as a major city. Both are located on the Atlantic coast which made trade easy. The two cities were big on industrialization also and also initially had an abundance of crop especially wheat due to the land which could easily be exported through its ports.
The Germans invaded Belgium which allowed them to attack France from the North. If successful, the Germans would move their armies East to fight Russia, avoiding a two front war.