The word "Flank" means the same in any war - it means the exposed sides of an army/navy vulnerable to attack by the enemy .
The term 'civil war' means a war between factions in the same country
In the US Civil War, the South wanted to secede from the Union. This means that they wanted to split off and form their own country.
Family feud. A country fighting against itself is civil war.
He has joined (or been drafted) into a specific unit and placed on their muster role.