The question stems from the historical notes I can find online stating that the 810th was activated on 26 June 1941, 2 months after the dedication of Macdill. Is it possible that the men who would compose that unit were already assembled or was activating a unit essentially the day se officers and NCOs show up to get the paperwork side of forming a unit done?

In the case of the 810th Engineer Aviation Battalion, a group of enlisted men from the 41st Engineer Regiment (General Service) plus four Regular Army officers had been selected and previously trained as the cadre that would be used to form the battalion. Draftees assigned directly from a reception center (they had been given their initial issue of uniforms and had their records initiated, but had not yet had any intensive military training) would form the balance of the battalion. The date these men arrived at the activation site coincided with the official activation date of the unit, although a unit often did not receive all of its enlisted fillers exactly in the prescribed timeframe (a certain amount of extra time was built into the individual training program to account for latecomers). The cadre would then guide the draftees through individual, unit, and combined-arms training, plus any specialty training or major maneuvers.

The cadre concept of using a small number of trained officers and enlisted men combined with a balance of new draftees plus a training program to form a complete unit ready for action is most well-known when it comes to infantry divisions. Units formed under this method have come to be known in modern historiography as "draftee divisions," vindicating this manpower decision by the Army, as they generally performed well. Regular Army divisions furnished the cadre for Organized Reserve divisions, who themselves once formed and sufficiently trained furnished cadres for other Organized Reserve or Army of the United States divisions. For example, the 2nd Infantry Division, a Regular Army unit, furnished a cadre for the 85th Infantry Division, ordered into active military service on 15 May 1942. The 85th then furnished a cadre for the 103rd Infantry Division, ordered into active military service on 15 November 1942. The enlisted fillers for the 103rd came from reception centers of the 4th, 6th, 7th, 8th, and 9th Service Commands, i.e., the states of Alabama, Florida, Georgia, Tennessee, Mississippi, North and South Carolina, Illinois, Wisconsin, Michigan, Colorado, Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota, Wyoming, Arizona, California, Idaho, Montana, Nevada, Oregon, Utah, and Washington.

The cadre for a new infantry division (that varied in size from 15,245 in November 1940, to 15,514 in August 1942 to 14,253 in July 1943) as dictated by the Army Ground Forces in March 1942 was initially 172 officers and 1,190 enlisted men, increased to 185 officers for divisions to be activated in June 1942 and after. For post-August 1942 divisions, the cadre was further increased to 216 officers and 1,460 enlisted men. Divisions, once activated, were also assigned an overstrength of 1,200 men at activation to somewhat mitigate the effects of themselves later providing a cadre, the personnel disruption from which it was estimated required about four months of additional training to recover from.

Lee, Ulysses. United States Army in World War II, Special Studies: The Employment of Negro Troops. Washington, D.C.: United States Army Center of Military History, 1966.

The 810th, with a cadre from the 41st Engineers and four Regular officers, had a well-spent if brief six months of training before leaving the New York Port of Embarkation. Its green men learned to operate heavy equipment by building roads, bridges, and fortifications, and by doing general construction work at their home station and at other new and expanding posts.

The companies of the 810th were activated separately and therefore trained and worked at different levels. Company A, in August 1941, built a practice bomb target at Mullett Key, a small island at the entrance to Tampa Bay, about twenty miles from MacDill by water. With its personnel and its four trucks, one compressor, two D-4 tractors, and one command car, it went out to the island, site of a ruined fort used during the Spanish-American War, and proceeded to clean up the flat, mosquito-ridden, swampy key. The men cleared the island of palmetto trees and brush, losing track of the number of rattlesnakes they killed though they preserved the skins to make shoes and belts for their wives and sweethearts. They repaired the island's long unused rain-catching equipment to provide a water supply. They acquired an old sixty-foot tug from the district engineer office, patched and painted it up, and operated it with their own crew, carrying supplies and men back and forth.

Other companies took over the Mullett Key project later. The men of these units acquired from their training on this subtropical island a conditioning and resourcefulness that was to stand them in good stead sooner than was then expected. At MacDill, one company participated in a local maneuver, testing theories of air base defense. Another constructed bombing targets and a drainage system at Morrison Field, Florida, and taxiways at the Charleston, South Carolina, airport. Still another, three weeks after activation, went to Lake Charles to take part in the Louisiana Maneuvers of 1941, working with a battalion of the 21st Engineer Aviation Regiment, a unit experimenting with new materials for airfield construction. This company later moved to Greenville, South Carolina, where it constructed camouflaged revetments for observation planes, and to Wilmington, North Carolina, repeating the job for fighter planes and building asphalt connecting taxiways to the revetments. With the help of Tampa citizens, elements still at MacDill Field provided their own recreation by taking an old post theater and converting it into a service club.

The companies at Wilmington and Charleston moved to Indiantown Gap, Pennsylvania, in January 1942 to prepare for overseas movement. Headquarters Company and Company A, 810th Engineer Aviation Battalion, still at MacDill, left on New Year's Day for Savannah where, in a tented area, in cold rain and red mud, they built a complete taxiway and hard-standings for the air base. Despite the difficulties of living and working at the new base, the tense atmosphere of the month following Pearl Harbor and the obvious necessity of the work kept the unit's morale high. The 810th as a whole was about as well trained in its six months as could be expected.

With one week's warning orders, the companies assembled at the New York Port of Embarkation. They discovered that they were "pretty much on our own" in making preparations, for no standard procedure had yet been worked out . for moving such a unit. Seventy flatcars of automotive and heavy construction equipment were made ready and shipped to the west coast. At New York, the unit bivouacked on the SS America, then being converted to a troop ship in the Brooklyn Navy Yard. After two days of loading and unloading ships, it boarded the USAT J. W. McAndrew, which, in a convoy of six other ships bearing the units of Task Force 6814, some of which were later to become famous as the Americal Division, departed New York for Australia on 23 January. On shipboard the 810th was joined by the relatively untrained 811th, which had spent its one month of service becoming acquainted with the experimental mats and equipment of the 91st Engineers in unusually heavy snow at Langley Field.