At first, if the USA were to divide along red and blue state lines you might not even notice any difference in your day to day life. The red area would create a new federal capitol, like Dallas for example.
There will be a draft, and huge armies will be raised. If you are over 40 don't worry.
I’m over 50 and have mobility problems, but by God, I’m not useless. If it came down to it, I can still wear a uniform.