If there is a Civil War, it will be nothing like the first one. Most likely it would consist of a few small conflicts such as coordinated riots, or paramilitary organizations fighting government or state troops- not an outright, nationwide conflict.- not two sides each with its own army slugging it out in major pitched battles. More like a sporadic guerrilla war, such as been happening in the Philippines for decades, or the conflicts in New Guinea.
If another World War does break out, you better believe the US will be the major player. N o doubt there. The US could find itself fighting North Korea, China and Russia, Iran, and of course the ongoing fight with Islamic terrorists.