The Civil War saw a US Army General, Robert E. Lee, defect to a cause he did not truly believe in, because he refused to fight against his home state. You might also want to consider how many veterans there are in the USA, that is, people with military service experience now living as civilians. Last census poll put that number at over 18 million, across all branches of service. Some might re-enlist to fight for the government, but you wouldn't even need 10% of them to go rebel to have a multi-million man rebel force with military experience and contacts within every branch to possible sympathizers. Absolute nightmare.