You're confusing
nation[1] with
state[2][3] (though you may be correct that it might be more accurate to say "overthrow a government"). The writers of the Constitution didn't envision a standing army (or at least not a very large one) which is why they a) gave Congress the power to raise armies and b) highlighted the importance of a "well regulated militia being necessary to the security of a free state."
"Security" meant repelling foreign invaders and stopping insurrections, not starting them. To provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions[4]
[1] https://en.wikipedia.org/wiki/Nation
[2] https://en.wikipedia.org/wiki/Sovereign_state
[3] https://en.wikipedia.org/wiki/State_(polity)
[4] https://en.wikipedia.org/wiki/Article_One_of_the_United_Stat...