Early firearms were hampered by their extremely low rate of fire (about one volley every two minutes):
Thus an infantry formation would only be able to fire once at an onrushing cavalry charge. The innovative Dutch commander Maurice of Nassau devised a solution. He restored the Roman practice of linear formations, drawing his men up in thinly-packed ranks (at most 10 men deep) of long lines. The first rank would fire, then retire to the rear to reload; then the second rank, now at the front, would unleash its volley and then perform the same maneuver. By rotating through lines, a Maurician army could theoretically sustain an almost continuous barrage.
Maurician tactics were demanding of the average soldier, now tasked both with performing coordinated actions with his comrades and standing firm in the face of enemy fire. The panacea was drill: practicing march and countermarch maneuvers. To facilitate this, Maurice divided his forces into smaller units and increased the ratio of officers to men. Companies of 250 with eleven officers were reduced to 120 men with twelve officers; regiments of 2,000 were replaced by battalions of 580. The diary of Anthonis Duyck, a member of the Dutch general staff, reveals a life spent constantly on exercises, supervising troops as they practiced forming and reforming ranks and marching in formation. These motions were codified by Maurice’s cousin John in an illustrated manual that sketched out how to use key infantry weapons. In 1599, Maurice also received sufficient funds to equip the Dutch army with firearms of standardized size and caliber. Standardization of uniforms followed.
It was not the Counts of Nassau, however, but rather Gustavus Adolphus of Sweden, who translated the ‘revolution in tactics’ into battlefield success. Thanks to extensive drilling, he improved his forces’ rate of fire until only six ranks were needed to maintain a continuous barrage.