Made me think about what is a division although we use the word quite often regarding WWII. It was always emphasized that it was a self contained unit with everything it needed being internal. The move from 4 to 3 regiments, (one in reserve) is very clear now in my history reading.
World War II[edit]
The divisional system reached its numerical height during the Second World War. The Soviet Union’s Red Army consisted of more than a thousand divisional-size units at any one time, and the total number of rifle divisions raised during the Great Patriotic War is estimated at 2,000. Nazi Germany had hundreds of numbered and/or named divisions, while the United States employed 91 divisions, two of which were disbanded during the war.
One notable change to divisional structures during the war was the shift from square divisions (of four infantry regiments) to smaller triangular divisions (of three infantry regiments). This was due to increases in mobility and the need to pare down structures to be as efficient as possible. The triangular division also fitted with the tactic of “two forward, one back”, in which two of the division’s regiments would be engaging with the enemy with one regiment in reserve.
All divisions in World War II were expected to have their own artillery formations, usually the size of a regiment depending upon the nation. Divisional artillery was occasionally seconded by corps level command to increase firepower in larger engagements.
Regimental combat teams were used by the US during the war as well, whereby attached and/or organic divisional units were parceled out to infantry regiments, creating smaller combined-arms units with their own armor and artillery and support units. These combat teams would still be under divisional command but have some level of autonomy on the battlefield.
Organic units within divisions were units which operated directly under Divisional command and were not normally controlled by the Regiments. These units were mainly support units in nature, and include signal companies, medical battalions, supply trains and administration.
Attached units were smaller units that were placed under Divisional command temporarily for the purpose of completing a particular mission. These units were usually combat units such as tank battalions, tank destroyer battalions and cavalry reconnaissance squadrons.
https://en.wikipedia.org/wiki/Division_(military)
A standard Soviet rifle division was considerable smaller then a U.S. Brit or German infantry division. At strength it numbered about 9,400 men. If it was a “Guards” rifle division it’s strength was about 10,500.