A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.”

The condition set in the opening phrase: A well regulated Militia, being necessary to the security of a free state, .  Sets the condition for the Right put forth in the closing phrase. As Madison wrote this in 1780 there was about to be no standing Army. The lack of a Standing Army let the Defense of The Nation fall upon the Citizens: in the form of 'Well Regulated Militia' which were fighting units that fought in the Revolutionary War, with Command Structure and pay. As our modern AI information portals will recover from records; the Well Regulated Militia was a vital part of American defense all the way up to and through WWII at the ending of which was written the The National Security Act of 1947. As quoted below this created a Standing Army, which with the 50 States Federalized National Guards make up the Force today that no longer rely upon the Citizen Militia. It is this Militarization of Federal and States Governments that has replaced the Second Amendments first clause of Necessary. Today civilian militia, well regulated or completed unregulated and run amok as gun deaths in the United States demonstrates, are to face the judgement of The People as whether they shall not be infringed. 

General Washington's Farewell

General George Washington addressed the troops in a farewell speech shortly before the disbandment. His farewell was a significant moment, as he expressed gratitude for their service and the sacrifices made during the war.

Key Dates

Date	Event
October 18, 1783	Congress proclaimed the discharge of troops enlisted for the war.
November 2, 1783	Washington delivered his farewell address to the army.
November 4, 1783	Official disbandment of the Continental Army.

It depends on what you mean by “large standing army.” The U.S. had a small regular Army for most of the 1800s; even in 1861 it had fewer than 17,000 soldiers, and the huge Civil War armies were mostly temporary volunteer forces, not a permanent standing army.

If you mean a large, modern force capable of operating at national scale, that emerged with World War I mobilization: after the U.S. entered the war in April 1917, the Army expanded to millions within about a year and a half. That was roughly 134 years after the Continental Army’s 1783 disbandment—but it was still a wartime expansion, not a permanently large peacetime force.

By your definition, the U.S. did not have a large regular army capable of defending the whole nation immediately after the Revolution. The 1784 force was tiny and mainly intended for frontier security; the country still relied heavily on state militia.

The War of 1812 made clear that this arrangement could not reliably defend the country. The Regular Army fought alongside hundreds of thousands of militia and volunteers, and the National Park Service describes the war as demonstrating that the nation could not depend on militia alone. So the closest answer is: the need for such a force was recognized by 1815, about 32 years after the Continental Army’s disbandment, but the United States did not then maintain a large peacetime army able to defend the entire nation without militia. National Park Service

The Army remained relatively small in peacetime for much of the 19th century. A truly large force came through mass mobilization for World War I, beginning in 1917—but that, too, was a wartime expansion rather than a large permanent peacetime force. Wikipedia

After World War I, the Army rapidly shrank rather than remaining a large force. Congress set the Regular Army’s authorized strength at 280,000 in June 1920, then cut it further; by 1927 it was described as little more than a token force. So the wartime army did not become a lasting, large peacetime force capable of defending the country on its own. ebsco.com

For scale, the U.S. entered World War I with just over 120,000 Army personnel and expanded to millions during the war—then demobilized quickly after the armistice.

Broadly, yes—but distinguish a permanent military establishment from a force large enough to defend the country on its own.

After World War II, the U.S. chose not to demobilize back to its earlier, very small peacetime force. The National Security Act of 1947 reorganized the military under a unified national-security structure, and the 1948 Selective Service Act preserved a peacetime draft system. Together, these mark the postwar shift toward a permanent, federally organized armed force prepared for sustained peacetime readiness and rapid expansion.

That does not mean the U.S. maintained a force that could meet every major threat without reserves, conscription, or mobilization. A more precise summary is: World War II established the precedent of mass mobilization; the postwar years made a substantial standing force and permanent mobilization system enduring features of U.S. defense. The 1947 act itself primarily reorganized the services; it did not, by itself, set a specific force size.

Right—the National Guard is an organized, trained reserve force, so it isn’t “civilian militia” in the sense we’ve been using. In that frame, the 1947 act marked a shift toward a permanent active-duty force, but the act itself mainly reorganized the services; it did not establish a particular force size or make the active-duty military independent of reserves.