In 1910 he rejoined his regiment in Tientsin China and was promoted to the permanent rank of major. He was then transferred to the 24th Infantry on Corregidor and was instrumental in creating the plans for the defense of the Bataan Peninsula. Following this assignment, he returned to the General Staff in Washington, D.C. and on the outbreak of the First World War was instrumental in drafting the Draft Act of 1917 and plans for an American Expeditionary Force. This work caught the notice of John J. Pershing, who selected Palmer to become his Assistant Chief of Staff for Operations, G-3, where he went to France and set up operational plans and staff schools for the American Army. He left the AEF staff due to illness, but recovered in time, as a colonel, to command the 58th Infantry Brigade of the 29th Division in combat against the enemy at Verdun in the Meuse-Argonne Offensive that helped to end World War I. He received the Distinguished Service Medal for his service in France.
Post–World War I
As the principal formulator of military policy following World War I, he was the guiding force in the creation of the National Defense Act of 1920. It was this Act which reaffirmed America's reliance upon the citizen-soldier for her defense and established the "Total Army" composed of the Regular Army, the National Guard, and the Army Reserve. For his accomplishments, he was made aide-de-camp to Army Chief of Staff John J. Pershing and promoted to permanent brigadier general in the Regular Army. He then completed his military career by commanding the 19th Infantry Brigade in Panama.
In retirement, General Palmer continued to champion the cause of universal military service. He wrote numerous books and articles about military policy. A strong advocate of the role of the citizen-soldier in the army of a democracy, Palmer diverged from the views of Emory Upton, with whom he is often compared as a great philosophical thinker-philosopher of the U.S. Army. He was recalled to active duty by Army Chief of Staff George C. Marshall just prior to the Pearl Harbor attack and served as an advisor on military policy to the War Department General Staff throughout World War II. Palmer was the oldest military member to serve during World War II.
Second retirement, death and burial
Palmer retired again soon after the end of the Second World War, receiving a second Distinguished Service Medal. He lived in Washington, D.C. where he died on October 26, 1955. Palmer was buried in Arlington National Cemetery, Section 34, Grave 50-A.
Family
In 1893, Palmer married Maude Laning, and they were the parents of a son, John McAuley Palmer IV, who died at age 6, and a daughter, Mary Laning Palmer. Mary Palmer was married to Norman B. Chandler, and after his death during World War II she married George H. Rockwell. Mary Palmer was the mother of Army Colonels Norman Palmer Chandler and John Palmer Chandler.
Decorations
Brigadier General Palmer's ribbon bar included amongst others :