Why was the city of Nanjing extremely vulnerable to Japanese attack?

Nanjing was extremely vulnerable to Japanese attack due to several factors, including its strategic location as the capital of China at the time, which made it a primary target for military conquest. Additionally, the city had inadequate defenses and was unprepared for a large-scale assault, as Chinese forces were engaged in a protracted conflict with Japan and had dispersed resources. The rapid advance of the Japanese military in 1937, coupled with internal political strife in China, further exacerbated Nanjing's vulnerability, leading to the infamous Nanjing Massacre.