Why was the city of nanjing extremely vulnerable to japanese attack? ?

The city of Nanjing was extremely vulnerable to Japanese attacks because Chinese political and military leader Chiang Kai-shek and General Chen Cheng withdraw from the city of Nanjing along with elite Chinese troops under heavy and constant bombardment by the Japanese Imperial Air Force . Thus the city was totally unprotected and totally vulnerable to the Japanese attack.

by 1890, the united states had by far the world's most productive economy. american industry produced twice as much as its closest competitor--britain. but the united states was not a great military or diplomatic power. its army numbered less than 30,000 troops, and its navy had only about 10,000 seamen. britain's army was five times the size of its american counterpart, and its navy was ten times bigger. the united states' military was small because the country was situated between two large oceans and was surrounded by weak or friendly nations. it faced no serious military threats and had little interest in asserting military power overseas.