martinithebun
FNP40
Your question is rhetorical, because this scenario has already happened and it was a bloodbath. See U.S. Civil War, 1861-1865. Before anyone chimes in and says that the Confederacy was a foreign country, that is not true from the Union point of view. The Confederate States were considered to be "in rebellion against the United States". The US government did not acknowledge them as an independant country.
I don't have the exact number, but half or more of the 1860-61 US Army officer corps sided with the Confederacy. I don't have any numbers for enlisted or non-coms.
My point is that while some left, others stayed. Not everyone currently wearing the uniform is a die hard constitutionalist, pro-2A, or libertarian.
This is not a knock on the men and women wearing the uniform, just, IMO, the breakdown of the demographic.
+1. truth.