Why did people sometimes call Liu Bei "明将军"?
It's an honorific, meaning &quot;illustrious&quot; or &quot;enlightened general&quot;, and as such is not specific to Liu Bei.
For example, in the Records of the Three Kingdoms, Pei Song's annotations cite a passage where Huangfu Li (皇甫酈) addresses Li Jue (李傕) as 明將軍:

近董公之強，明將軍目所見...
In recent times, you yourself, illustrious general, saw with your own eyes how powerful Dong Zhuo was ...