After soldiering on for years with a polyglot army of foreign-supplied troops and mercenaries, the Dutch took steps to reform their armies starting in 1590 under their captain-general, Maurice of Nassau, who had read ancient military treatises extensively.
For many years, the Dutch army was comprised of soldiers from various nations and mercenaries.  In 1590, under the leadership of Maurice of Nassau, they began to reform their military structure. This reformation was inspired by his extensive study of ancient military treatises.