Although conceivably, a young man from Florence could buy arms and armor and set off to join a mercenary company, for the most part a mercenary for hire was a very specific kind of person.

Initial mercenary companies were more often than not composed of veterans of fighting in France and/or Germany, where dynastic fighting had blurred the lines who precisely was fighting against who, and individual vassals and the forces they commanded opportunistically backed different sides based on plunder, risk/reward, and who seemed to be winning. So most early soldiers for hire were mounted men-at-arms; men who had been born into landowning families but perhaps stood to inherit little more than a horse and a name. These men were quick to attach themselves to powerful landed lords, offering their service in battle in exchange for lodgings and a place in their retinue.

So first recorded company for hire in Italy in the early 14th century was employed first by the Marquis of Monferrat in northwestern Italy (unsurprisingly near France). The Monferrat was threatened on the west by the Counts of Savoy and to the east by the ruthless Visconti of Milan. Apart from calling his knights to arms to repel incursions by both the Milanese and the Savoyards, the Marquis Giovanni also hired what amounted to a band of Hundred Years War veterans living as outlaws somewhere in southern France. They were commanded by a German, Alberto Sterz, and an Englishman, John Hackwood. They numbered about two thousand men at arms, who rode to battle but "in the english fashion ("all'Inglese") dismounted to fight (probably due to fear of being unable to replace horses). They were probably also accompanied by archers armed with longbows.

The later appearance of Italian armed bands for hire can be attributed to a sort of snowball effect, as commanders and veterans loyal to them accepted new contracts for employment instead of returning home. At the late date of 1377, Alberic of Barbiano established the first Italian-only mercenary company. Alberic himself was a son of the Count of Cuneo, a vassal of the Marquis of Monferrat, and was most probably present when his father was called to arms in the conflict between Monferrat and Savoy. Instead of returning home at war's end, Alberic, then sixteen, followed John Hackwood and Alberto Sterz as they traveled south to fight for Pisa, a Tuscan city at war with Florence. At this point (the 1360's) the company, called "The White Company" numbered about four thousand knights and two thousand footmen.

The proliferation of armed bands soon created a system by which in the 15th century war became a kind of "Theatre" in which campaigns were waged in a (very short) summer campaigning season as a game of figures and hypothetical rather than actual confrontation. Whichever side could hire the most mercenaries usually won, with individual companies regularly switching sides depending on who was paying better, who was winning, and other criteria. In addition, even when armies were within striking distance few actual pitched battles were fought; maneuver and positioning became exponentially more important as captains tried to keep their companies intact.

When two armies finally gave battle, the encounter was often short and relatively bloodless; as soon as one side worked out an advantage, the "loser" often attempted to swiftly withdraw in good order (often to the frustration of their employers; "They avoid defeat just as long as they avoid battle" wrote Macchiavelli).

Sieges were more common than pitched battles. Between the 13th and 15th centuries, the Milanese, Paduans, Veronese and Florentines were almost continuously involved in some kind low-level warfare, often bullying smaller surrounding towns into submitting in order to establish a ring of fortified burghs around their capital. Oftentimes, these sieges were perpetuated by the mercenaries themselves, who needed to be occupied lest they start pillaging; perpetuating a vicious cycle by which larger, richer cities were constantly hounding out weaker ones.

If you'd like to read more about the decline of Mercenary Warfare in Europe at the hand of professional armies of Early Modern States do pick up "War and Society in Renaissance Europe, 1450-1520" by John Rigby Hale (Johns Hopkins University Press, 1986). If you have a JStor account, Micheal Mallet's paper "Diplomacy and War in Later Fifteenth Century Italy" could also be helpful.

There is great writings about this very topic by a German mercenary by the name of Peter Hagendorf. He wrote a great account of his time fighting as a mercenry in the 30 years war.

While all wars are in some way said to bring out the worst in mankind, the Thirty Years war, as Historian Peter Wilson writes “became a benchmark of brutality for centuries”. Considered by Germany as worse than both World Wars and the Black Death as the greatest disaster to ever hit their country – It goes without saying that the Thirty Years war holds a high watermark as to the sheer disregard for human life and the amount of blood shed. We can see this reflected throughout the Writings of the Mercenary, Peter Hagendorf. Written with a sense of disassociation not uncommon in military men who have seen their fair share of cruelty on the battlefield, he describes the scene in which a village of what he estimates to be a thousand peasants burnt to the ground “The peasants in the churchyard there resisted so forcefully that we could achieve nothing without cannons. Thus we went back, for there were one thousand in there. Yet we set fire to their village and let it burn” - This was not, as we can see from other historical accounts, an isolated event, as by the end of the war, Swedish armies alone would have laid waste to 2,000 castles, 18,000 villages and 1,500 towns in Germany – One third of all German towns that existed at the time.

This raw violence could perhaps, at least in part, simply be due to the magnificent vastness of the armies involved in the fighting. In one of Hagendorf’s accounts, he describes a battle in which the French army, 70,000 men strong arrived to meet his, which in turn stood at 120,000 thousand. It is evident then, with such epic fielding of armies rarely seen before this time, at least with this amount of frequency, how 25-40% of the population in the German territories were wiped out over the course of the war. This violence naturally had a significant effect on the psyche of the men who fought in these monumental armies. Again, taking from Hagendorf’s writings, many soldiers took to alcohol to, at least temporarily, forget about the death and destruction occurring on a daily basis around them. In the writings of the Mercenary, barring the mentions of interactions with his family, consumption of alcohol is probably mentioned more than anything else the soldier did in his spare time. Instances of “Getting well and truly drunk”, being too hungover to keep up with his unit along with a general infatuation with wine are very evident in the document. One can deduce then that, alcohol was a major factor in allowing the men of these armies to hold their nerve, or at least, dissipate the natural feelings of panic, fear and guilt one would most likely suffer in the conditions of this war.

This is not to say that Hagendorf did not partake in some of the actions that makes this war stand out as being notably brutal. He accounts of entering villages after successfully sacking them to take ‘booty’ – Such as in the Spring of 1634 where his spoils were “A pretty Maiden and 12 talers’ worth of old, clothing and linens” and in October of 1634 wherein he “Led away a young girl, but I let her go again […] Which I often regret, because I had no Woman at this point” Hagendorfs plundering was by no means exclusive to him, as it was the standard for armies attack a town/village to plunder, rape and generally take what they wanted, but it is through Hagendorf’s accounts we can see the mindset of the average soldier partaking in this pillaging, where it seems that the victorious soldiers held a sense of entitlement to anything or anyone in the unfortunate village being ransacked, and this was their reward for a hard fought battle.

Examining these historical accounts, it is easy to imagine the men of these fighting armies to be devoid of any sense of humanity, killed slowly by the accumulative terror of the atrocities occurring around them, or perhaps it was never present in some of these men in the first place – But we can see in the diaries of Hagendorf, at least small glimpses of it when he writes about his family. The practice of a fighting man’s family following him from battle to battle was not an uncommon one. There are many reasons for this – Firstly being practical, as it was advantageous for a soldier to have his wife and children tend to duties for him, such as mending his wounds. Along with this, one must assume that given the nature of the war, a soldier having his family follow him across Europe was practical for another reason in that, there may not be a home to return to when the war finished, and perhaps the safest place to be, almost counter-intuitively, was on the tails of an advancing army.
Another reason for this practice was the emotional effect it must have had on the soldiers. Men seeing their families would allow them to remember just exactly what they are fighting for, maybe giving them a boost of morale – And while this might not have happened in the case of Hagendorf, as he was a mercenary who fought for money rather than defence of his nation, it could have still gave him some type of heightened reason to fight, or rather survive, in battle as his family and their survival most likely depended on him.

Surviving however, might have been a little bit easier for Hagendorf, at least compared to his peers who fought in the national armies. As we can examine from Hagendorf’s writings, Mercenaries cared little for what side they actually fought for in this war. And regularly changed sides, as is evident when his regiment was defeated by the Swedish army “So we had to surrender, because we had no hope of any relief […] We had to allow ourselves to be enlisted into the Swedish Army”. This practice was not uncommon during this war, as it was more in the benefit of victorious armies to add to their ranks than to needlessly slaughter fighting men who were fit to fight.

In conclusion, it can be seen through the account of this Mercenary many things about how war was waged during this time. Firstly, just how little value was placed on human life, with many villages and towns being destroyed for very little reason other than the capture of ‘booty’ and simply because they stood in the way. It can also be seen how this violence effected the men who partook in it, often resulting in cold detachment from the reality of just how barbaric the conflict around them was – Perhaps this is what allowed them to plunder, rape and pillage in such ways that they did?

(Sources: Helfferich, Tryntje, The Thirty Years War; A Documentary History (Cambridge, 2009) John Hearsey McMillan Salmon, History of Europe (2015), available at Encyclopaedia Britannica, (http://www.britannica.com/topic/history-of-Europe) (30 December 2015) Medick, Hands and Marschke, Benjamin, Experiencing the Thirty Years war (New York) Philip Kaiser, Darrel, Origin & Ancestors Families Karle & Kaiser of the German-Russian Volga Colonies (Texas, 2006) Wilson, Peter H., The Thirty Years War; Europe’s Tragedy (Cambridge, 2009))