I'm extremely curious about this! If I remember correctly, during the invasion of the Philippines that a US Army general surrendered the entire garrison that was defending the Philippines to the Japanese and there happened to be Marines on the island who were stuck with the outcome, but my question is more or less in terms of actual combat operations during WWII. I also know that a handful of Marines used the Army Air Corps to drop behind enemy lines around the time of D-Day, but they didn't have any command structure. I also know that Marines were slated to be the QRF for the Rangers at Pointe Du Hoc and were told to stand down because of the Marines reputation from WW1 in the Battle of Belleau Woods.
What I'm curious to know is, if at any point in time, did the US Army whether it's at the battalion, brigade, or division level have any Marine Corps units (say, a company) attached to them, or vice versa, during any combat operations in the Second World War.
Thanks!
A few occasions, including some of the reverse of what you speak of, from both world wars.
During World War I, the U.S. Army's 2nd Division was organized on 26 October 1917 in France, from units that had arrived piecemeal in the country and that were originally destined for other roles, and was uniquely made up of both soldiers and marines. The 5th Marine Regiment had sailed for France under a promise from Secretary of War Newton D. Baker that Marine units would see combat with the Army (even though General John J. Pershing personally did not see any use for them, and proposed that they "be converted to Army troops"). After the Departments of War and the Navy made an arrangement for two regiments of soldiers and two of marines to fight alongside one another in a division, the 6th Marine Regiment was also sent to France. The 2nd Division, organized like a normal Army square division, had one Army infantry brigade (the 3rd, of the 9th and 23rd Infantry Regiments and the 5th Machine Gun Battalion) and one Marine brigade (the 4th, of the 5th and 6th Marine Regiments and the 6th Machine Gun Battalion), with the rest of the division being manned with Army troops. The 2nd Division's first commander was a Marine major general, Charles A. Doyen, as was its last before it was inactivated, John A. LeJeune. Two Army major generals, Omar Bundy and James G. Harbord, also commanded the division in the interim during the war.
During World War II, there were two notable occasions where Army troops served under Marine leadership and vice versa. Late in the Guadalcanal campaign, both Army and Marine units on the island were beaten down from constant combat, lack of replacements, and the jungle conditions. On 12 January 1943, the commander of the U.S. Army's XIV Corps, Major General Alexander Patch, formed the CAM (Composite Army-Marine) Division from the headquarters of the 2nd Marine Division, the 6th Marine Regiment, the 182nd Infantry Regiment of the Americal Division (itself a unique unit, made up of three "orphaned" National Guard infantry regiments sent to defend New Caledonia, whereupon they were formed into a division with other units), the 147th Infantry Regiment (a separate unit), and Americal Division and 2nd Marine Division artillery units. The "division" operated on the island until the Japanese withdrawal was completed on 7 February 1943. As operations in the Pacific accelerated in mid-1944, the U.S. Army activated the Tenth Army, the last field army-sized formation to be activated during the war. For Operation Iceberg, the invasion of Okinawa in April 1945, the Tenth Army, commanded by Lieutenant General Simon Bolivar Buckner, consisted of the Army's XXIV Corps (the 7th, 27th, 77th, and 96th Infantry Divisions) and the III Amphibious Corps (the 1st, 2nd, and 6th Marine Divisions), with a joint Army-Marine air element attached. After General Buckner was killed by an enemy shell burst while visiting the front lines on 18 June 1945, Marine Major General Roy Geiger assumed command until General Joseph Stilwell, commander of the Army Ground Forces, could be recalled from Washington. Geiger is the only Marine officer to have commanded a field army-sized force.
Sources:
Appleman, Roy E., James M. Burns, Russell A. Gugeler, and John Stevens. United States Army in World War II, The War in the Pacific, Okinawa: The Last Battle. Washington, D.C., United States Army Center of Military History, 1948.
Miller, John, Jr. United States Army in World War II, The War in the Pacific, Guadalcanal: The First Offensive. Washington, D.C., United States Army Center of Military History, 1949.
Wilson, John B. Maneuver and Firepower: The Evolution of Divisions and Separate Brigades. Washington, D.C., United States Army Center of Military History, 1998.