Short answers, pulled from the story.
The incident began when two Japanese naval officers, Naval Sub-Lieutenant Oyama and his driver Saito Yozo, were shot dead near Hongqiao Airport. This confrontation with Chinese guards from the Peace Preservation Corps triggered immediate diplomatic fury and demands for troop withdrawals.
Black Saturday occurred on the 14th of August 1937 when errant bombs intended for a Japanese flagship exploded within the Shanghai International Settlement. Casualties reached 3,000 people with between 700 and 950 civilians killed outright at locations like the Great World entertainment center.
Chinese troops withdrew from downtown Shanghai by the 26th of October 1937 after Dachang fell to overwhelming force. The position was reduced to rubble by 700 artillery pieces and 400 aircraft following intense house-to-house fighting that changed hands up to five times daily.
Imperial forces confirmed deploying poison gas at least thirteen times during the battle according to official records. Units such as the 159th division reported enemy forces using tear gas and poison gas to掩护 infantry advances while their own units could not fire due to chemical exposure.