peter2010 wrote:... Was it a German invention?
Interesting question. As a wlld guess I'd say not. With 4,000 years of formal military history preceding it is counter intuative the technique would not appear until the 19th or 20th Century. At the start of the American Civil War Gen Scott outlined a plan for each regular US Army regiment to provide a 'battalion' cadre upon which the newly raised militia regiments would fall in upon for a few weeks of training. The regulars were to give all ranks from the militia colonel down to private instruction in all aspects of battalion operations. The plan was stillborn when Scott was pushed aside & organization of the Federal armies placed in other hands. In WWII the US Army had 'school' or training units that were organized & equipped as any combat unit, but which remained at the training sites in the US specifcally for the advanced or specialized training of soldiers for that type of unit. In the 1930s the US Army 2d Division functioned as a test unit for the various proposals for reorganization of the division, predominatly the triangular division organization.
I also seem to recall the last Century of the rear rank in each Roman Legion as the training company for the new recruits.