They're really different. Tibet was pretty much taken by the Nationalists during the early 1930s. The PRC further secured Tibet during the resumption of their civil war after WW2. Controlling Tibet would establish a better defensive position against the West (namely, the UK)--who were and would definitely use Tibet to exert leverage against PRC (e.g. by exporting weapons into Tibet, subsidizing their military, etc.).
I generally view the taking of Tibet as a move for securing loose ends against a generally hostile enemy (which the West turned out to be for 20+ years). I'm not sure if this is true, but according to Wikipedia, the Sino-Tibetan war (1930-1932) began when Tibetan forces invaded some Chinese warlord's territory, and that warlord was allied/in cahoots with the Nationalist Party (Chiang Kai-shek and Friends).
http://en.wikipedia.org/wiki/Sino-Tibetan_WarThe American Civil War was about the federal government consolidating its hold over the State governments. It wasn't in response to foreign threats.
The two events are similar in that all states seek to expand power, but that's as informative as saying all people eat, shit, and sleep until they die.