The first example of combined arms can be seen in the Swedish army during the 30 years war. The unprecedented interaction between infantry, musketeers, cavalry and artillery that king Gustaf Adolf of Sweden introduced, supposed the beginning of the modern warfare.
Until the 17th century, tactis were still much alike in the late middle ages. Blocks of infantry and cavalry acted separately, while artillery remained an inmobile, heavy unit for support or siege. The cavalry itself had lost the role of stormtroop it had before, as it relyed on heavily armored horsemen that performed shy movements of pistol attack and retreat, called "caracole". The infantry formation was the "tercio", a bulk group of pikemen surrounded by arquebusiers, where the long spears of the first provided protection to the latter. As early firearms were short-ranged and slow to charge, this conservative structure showed a good performance to the Spanish Empire at the time, in spite of confering warfare a complete lack of offensive. With the introduction of the field artillery, it became a very vulnerable target.
The Gustaf revolution
Gustaf had developed a new army, where mobility became the strongest feature. As a first measure, the arms and armors of the soldiers were significantly reduced, even to just light harnesses in the case of cavalry. Pikes were reduced to 3 metres (instead of 6), and muskets were made lighter, so that "fork" was not needed anymore.
The tercios of the Spanish tactical school were abolished. Gustaf's infantry instead used linear formations, much more mobile by the quick ordering from line to column. Musketeers could all fire at the same target and quickly alternate different lines of fire. Their tactics also changed: they usually shooted in salvos, instead of individually, provoking a psychological effect on the enemy. Right after the shot, a charge of pikemen (with their new maneuvrable pikes) crushed the weakened enemy. This combination gave back the infantry the offensive role it had lost.
The Swedish cavalry also favored the shocking effect of the charge. The caracolle was ineffective since the lethality of the pistol was low. Gustaf's horsemen instead advanced on the trot firing their pistols and then charged at full speed with drawn swords, especially vulenrable targest such as enemy cavalry in caracolle retreat or field artillery. The cavalry was also supported by units of musketeers granting it more deffensive capability.
The greatest improvement was that of the artillery, though. Gustaf and his artillery chief, Lennart Torstensson, decided to create a type of light artillery, of pieces of "only" 150 kg (while 1500 kg of the traditional cannons). This new artillery could perfectly follow infantry and even cavalry displacements, thus acting as a short-range, effective support at all time. These units of light artillery, moreover, were not assigned in an exclusive regiment. Instead, each infantry or cavalry regiment was provided with several pieces of artillery to the order of the same commander. This apparently simple issue proved to be essential, as this case of splitting had never been the case before.
The result? The German Catholic League was massacred at the battle of Breitenfeld, and the time of the Swedish Empire started. The old military tactics were suddenly abandoned and remplaced by Gustaf's ones. Napoleon itself considered Gustaf as one of his military inspirators.