Final answer:
Source A explains why the League of Nations lacked authority when Japan left. It lacked a military arm and could not enforce its rulings, which made it powerless against aggressive acts from member countries. The inability to limit member countries' activities led to the League's lack of authority.
Step-by-step explanation:
Source A provides insight into why the League of Nations lacked authority in regards to Japan leaving. The source explains that the League of Nations had no military arm to physically intervene in a member country's actions, which made it powerless to enforce its rulings.
For example, when Japan invaded Manchuria in 1931, the League could only issue protests and economic sanctions, but these had little impact and were not upheld by all countries. This lack of real power and inability to compel member countries to limit their activities contributed to the League of Nations' lack of authority and effectiveness.