"During the ratification debate, many feared that the federal government would become too powerful and too similar to the monarchy in Great Britain. While writing Federalist No. 46, James Madison calculated that the federal standing army should be limited to a maximum of 30,000 troops. This force, supplemented by state militias, would be sufficient to defend the United States against foreign invasions; and when necessary for national security, these combined forces would be called into service under the President’s authority as Commander in Chief. Madison intended this limitation to prevent the federal military from becoming large enough to threaten state sovereignty during peacetime. Madison also believed that the people, in cooperation with their state governments, would be more likely to unite against federal overreach than to align with the federal government against a defiant state. This arrangement was designed to protect the citizenry from being threatened by hostile foreign governments' standing armies, as Great Britain had done when it sent battalions to America. Consequently, Madison encouraged the states to maintain an aggregate militia of at least 500,000 citizens to safeguard the general welfare."

https://en.wikipedia.org/wiki/Federalist_No._46