Warmatrix

In the 17th century the strength of Sweden rose to the height of a major European power. This development was for a large part due to reforms in the army, instituted by the king Gustavus Adolphus.
Gustavus Adolphus improved on the line infantry tactics that had been pioneered by Maurice of Nassau. He deployed his musketeers only six lines deep. The first rank would kneel, the second crouch and the third stand up. They were trained to fire a simultaneous salvo, rather than just the first rank, as was the norm in Maurice's army. Once done, they switched place with the three hind ranks. He also equipped them with lighter muskets and made reloading easier by supplying paper cartridges. All this enhanced the firepower of the infantry. Yet pikemen were not dispensed with; about 50% - 60% of the infantry were musketeers, the rest pikemen.
Other parts of the army were revised too. Experiments were conducted with a light 'leather' cannon, but this proved ineffective and was soon abandoned. Instead the king settled on 3-pounder and 4-pounder cannons that were light enough to be transported across the battlefield during the battle, creating the first mobile field artillery. They could be drawn by a single horse, operated by two to three men and reached a rate of fire that was comparable with that of the musketeers. Heavier cannons, 6-pounders, 12-pounders and 24-pounders, traveled in the rear and were reserved for sieges, or deployed at the flanks.
The cavalry discarded the then common tactic of trotting up to the enemy and fire multiple rounds, the 'caracole'. Instead, it was trained to fire one or two salvos and then follow up with an aggressive charge with sabers. The front ranks of the infantry were ready to support them with a charge of their own, wielding their guns as clubs and also using pikes, while the rear ranks stood guard to deliver more gunfire.
The marching speed of the army was increased by reducing the baggage train and instead relying more on supply dumps established along the road. The troops were trained in all arms, so that infantrymen could ride horses, cavalrymen fire guns or make some other switch. All the improvements combined created an army where one branch was not more important than the other, but all worked together - the combined arms principle. To top it off they were inspired by patriotism and (protestant) religious zeal.
The standard tactical unit on the battlefield became the brigade, about 1,800 - 2,000 men strong, divided into 3½ regiments. Each regiment was supported by two light cannons. Cavalry was deployed in smaller units of 250 men, each supported by a single company of musketeers. Later in the 17th century CE, the number of musketeers in the infantry was increased and the number of pikemen decreased. The battalion became the main tactical unit, the brigade just a grouping of battalions.
Not all improvements were entirely new. Line infantry was pioneered by the Dutch under Maurice of Nassau; shock cavalry had been used by the Hugenots before. Neither were they fully the product of innovation. Most were born out of necessity. The smaller regiments were a result of a shortage of soldiers in the thinly populated Sweden; reduced armor and using sabers came from a lack of heavy armor and pistols, because the Swedish industry was still small. But to his merit, Gustavus Adolphus turned these disadvantages into advantages and united all into a combined arms force.
The revised Swedish army was successful against Russia and Poland, despite the superior cavalry of the latter. Sweden scored its most celebrated victories in Germany in the Thirty Years' War, though the campaign was a strategic failure and cost the life of Gustavus Adolphus.
The Thirty Years' War made Sweden one of the great powers in Europe for about a century. Other armies picked up the innovations by Gustavus Adolphus and further improved on them. The Prussian army is a prime example.