Beginning around 1909 and leading up to World War I, Capt. Harry Hand, a veteran of the Spanish-American War, drilled about two dozen young men in marching, handling arms, and fundamental Army tactics. Their troop, called the Bergen Guards, took part in many local parades but disbanded when the United States entered the world war. Harry Hand would later serve as borough clerk and as president of the board of education. At the September 1, 1919, Labor Day exercises at Recreation Park, 69 Allendale residents were given world war medals, including eight who had sacrificed their lives. Word War I veterans organized the Allendale American Legion Post No. 204 on December 16, 1919. First officers were Comdr. Maj. R.W. Rodman, Vice Comdr. George Buhlman, adjutant George Etesse, treasurer Russell Mallinson, and chaplain Edward Rouse. The membership committee included chairman Raymond P. Arlt, Oliver Asten, Herbert Winter, and R.V. Wall.
From 1947 to 1961, the American Legion headquartered in the former school on Franklin Turnpike, modifying it and renaming it the War Memorial Building. With declining membership, American Legion No. Post 204 surrendered the building back to the board of education, who then offered it to the borough in 1961. Local Veterans of Foreign Wars Post No. 10181 was established in 1988.
As of 2020, three plaques on the monument at Memorial Park list the names of Allendale residents who made the supreme sacrifice for their country. They are, from World War I (1917–1919): Harold Cook Ackerson, Marshall Harley Couch, James Robert Hubbard, John Raymond McDermott, Gustave William Nadler, Charles Larrett Nidd, Edward Sherrard Nidd, and Harry Otto Weimer; from World War II (1941–1945): David L. Ceely, John J. Fox, Edward J. Hamilton, Eugene A. Ivers, Bruce S. MacIntyre, John A. Sawyer, Harold W. Scott Jr., and Charles A. Yeomans; and from Vietnam (1969): Rocco J. DeMercurio. On June 2, 1983, Memorial Drive was renamed DeMercurio Drive in his honor. Many of Allendale’s streets are named after these heroes.