Completely and utterly false. Post-WWII was the ONLY time in history we didn't almost completely demobilize our military following the conclusion of hostilities.
The U.S. Army as a permanent institution began on 3 June 1784, when the Confederation Congress approved a resolution to establish a regiment of 700 officers and men. Intended as a force to assert federal authority in the Ohio River Valley, the regiment deployed at a string of posts along the Ohio where it functioned as a frontier constabulary during the last years of the Articles of Confederation era.
Congress adopted this tiny force after the reorganization of the government under the Constitution in 1789. Responding to the outbreak of Indian war in the Old Northwest—and especially to St. Clair's defeat in 1791, the worst setback at Indian hands in the army's history—the government expanded the military establishment to over 5,000 in 1792. Organized as the “American Legion” and commanded by Maj. Gen. Anthony Wayne, the army defeated the northwestern tribes at Fallen Timbers in 1794. During the same year, in response to European threats, the government launched a program of seacoast fortifications and added a corps of artillerists and engineers to build and man them.
The army became the center of intense partisan controversy with the rise of political parties and conflicting ideologies. Federalists sought to maintain a relatively large regular force, while Democratic‐Republicans opposed a sizable standing army that might require high taxes and threaten liberty. The result was a period of extreme instability in the army's size and structure. In 1796, the government reduced the army to 3,359. Two years later, however, the Undeclared Naval War with France led the Federalist Congress to expand the authorized level to over 14,000. Alexander Hamilton, appointed as inspector general and de facto commander of the army in 1798–99, strove to transform this force into a permanent, European‐style standing army, capable of checking domestic opposition. This political role aroused intense suspicion, and in 1800, following the diplomatic settlement with France, Congress reduced the army to 4,436. After Thomas Jefferson and the Republicans won the election of 1800, they fixed the peace establishment at two regiments of infantry, one of artillery, and a tiny U.S. Army Corps of Engineers—a total official strength of 3,287. In 1802, they also established the U.S. Military Academy at West Point, New York, primarily to train future officers in military engineering.
Throughout Jefferson's administration, the War Department and the small regular army performed a variety of constabulary tasks: administering the Louisiana Purchase; regulating Indian‐white relations; conducting diplomatic relations in the Spanish borderlands; and policing the Embargo Act against Great Britain and Napoleonic France. Meanwhile, deteriorating relations with Britain and France caused the Republicans to reassess their traditional antimilitarism, and in 1808, Congress authorized an increase to 9,921 officers and men. The onset of the War of 1812 continued the buildup, as the inadequacy of the militia for offensive operations left President James Madison little alternative but to expand the regular forces. The army's official authorized size reached 62,674 in 1814, although actual troop strength fell well short of this level.
The War of 1812 marked a major transition in the army's history. Until then, its dominant characteristics had been fluctuating size and organization, a high rate of turnover in the officer corps, and the absence of a clear sense of mission—conditions reflected in the poor military performance of the early war years. By 1814, however, the army's performance was improving, largely because of the rise of young, combat‐proven commanders to high and middle rank, exemplified by Jacob Jennings Brown, Winfield Scott, and Alexander Macomb.
Although Congress cut the army to 12,383 in 1815, many veterans remained in service, and they came to share a conviction that the army's chief mission should be preparation for a future war with a major European power. With the support of the Madison and Monroe administrations, they rationalized military management through permanent general staff bureaus, adopted uniform tactical manuals and regulations, launched a new and more systematic program of coastal fortification, and, under the direction of Capt. Sylvanus Thayer, revitalized the U.S. Military Academy. When Congress reduced the army to 6,126 in 1821, it tacitly followed a plan proposed by Secretary of War John C. Calhoun that called for a cadre organization: the retention of a high ratio of officers to enlisted men as a way to preserve military expertise and provide a framework for a rapid and efficient expansion in case of war (Skelton, 1992).
The reduction of 1821 was the last major cutback and reorganization of the army's basic establishment in the nineteenth century. It left a force of eleven line regiments under a major general with the title of commanding general of the army (a position held by Winfield Scott from 1841 to 1861), supported by a group of general staff bureaus—quartermaster, engineers, subsistence, ordnance, medical, and pay—reporting directly to the secretary of war. During the decades that followed, the army was usually dispersed at small garrisons along the frontiers and the Atlantic seaboard, where it continued to perform its customary constabulary duties. In particular, regulars enforced the Indian trade and intercourse laws and served as the government's principal instrument for conducting Indian removal. The latter duty produced one of the army's most most tragic assignments—the removal of the Cherokee Indians in the so‐called Trail of Tears (1838–39)—and the army's most frustrating experience of the antebellum era—the long guerrilla conflict in Florida of the Seminole Wars (1818, 1835–42, 1855–58).
The demands of national expansionism brought occasional increases in army strength, including the reintroduction of mounted regiments in 1833 and 1836, the first since the War of 1812. With the outbreak of the Mexican War in 1846, the army's basic establishment swelled to 17,812, achieved mainly by filling the understrength units with recruits; Congress supplemented this force with 10 temporary regular regiments and over 70,000 citizen‐soldiers raised as U.S. Volunteers. Although the postwar demobilization left the army at 10,317, the occupation of the newly acquired western territories soon renewed the buildup. The government added 4 permanent regiments in 1855, bringing the total to 19; on the eve of the Civil War, the regular army's actual strength stood at 16,367 officers and men.
16,367 men on the eve of the US Civil War that would see hundreds of thousands of men pitted against each other in single battles!