Correct, the American Civil War was not about the Second Amendment.
However, it did allow the southern soldiers to be armed before the government could secure a supply. In fact even after the government was established, most Confederate soldiers still used their own weapons.
The Spanish Civil War was also not about the right to bear arms, but again, it allowed the nationalist to defeat the socialists using their own firearms in the beginning.
The point is that it is better to be able to fight back than to be eliminated as happened in Cambodia, China, or the Albanian Genocide.