Complex sentence: The defense of Dachang was vital to how long the Chinese army could continue fighting in the Shanghai war zone; for this, Chiang Kai-shek mobilized whatever remaining troops he could find.
Split into simpler sentences: The defense of Dachang was vital to how long the Chinese army could stay and continue fighting in the Shanghai war zone. For this, Chiang Kai-shek mobilized whatever remaining troops he could find.