Losses in frontline infantry units during the summer and fall of 1944 were much more severe than anticipated. The War Department had estimated that 64.3 percent of Army Ground Forces battle casualties occurred in the infantry, and so planned the training of proportions of replacements at infantry replacement training centers accordingly. The War Department soon realized this estimation was far too low; it was raised to 70.3 percent in April 1944, and it was soon estimated (July 1944) that 90 percent of AGF casualties occurred in the infantry. In September 1944, ETOUSA was reporting over-strengths of men in every personnel category except infantry, with the replacement pool holding 119,000 men while being authorized only 70,000, but having a shortage of 7,000 riflemen. In October, replacement strength swelled to nearly 200,000 men; this also included replacements "on order" for October and November and those men in transit, on the basis of a maximum of 170,000 as prescribed by the War Department. Unfortunately, many of these men were actually Army Air Forces troops, men earmarked for activation of new units, or undergoing retraining and not suitable for use as infantry. This, along with shoddy counting methods, (the War Department at the end of September 1944 claimed a theater over-strength of 49,000 or even 69,000 men based on the approved amount, in the case of the latter figure counting "former" replacements, and those assigned as reserve to various units) brought the actual number of troops that were physically present in the theater down to 125,000, of which only 73,000 were suitable for infantry replacement purposes.

After their training at infantry replacement training centers in the United States, men destined to be replacements in Europe were received at ports in England or France and doled out to each field army and assigned to army and corps replacement depots or battalions as needed, to be selected in groups by those divisions that needed them. A problem with this process is that men were torn away from any friends they had made in training, and as a result were often scared and isolated. Stays at replacement depots could vary from a few days to a few weeks depending upon the tactical situation, and few replacement depots implemented any kind of program that would upkeep men's training; some even forgot how to maintenance and load their rifles. In units frequently restocked with replacements, an attitude of "I don't know him and he's probably just going to get killed anyway, so why should I bother to learn his name?" developed among the more experienced troops. In December 1944, the term "replacement" was dropped from official use in favor of the term "reinforcement". In March 1945, General Joseph Stilwell proposed that replacements be shipped in groups and be earmarked for assignment to specific units while still in the United States. ETOUSA rejected the "earmarking" proposal but accepted the other part of Stilwell's plan; replacements would be assigned four to a squad, with four squads making up a platoon and four platoons making up a company. Companies and platoons would be broken up as needed during assignment, but the basic unit of four men would always stay intact.

Sources:

U.S. Army in World War II: Logistical Support of the Armies September 1944-May 1945 chapter XI, by Roland G. Ruppenthal

U.S. Army in World War II, by Richard C. Anderson