From The Making of Eastern Europe: From Prehistory to Postcommunism, by Philip Longworth (Lume Books, 2020), Kindle pp. 269-271:
This malaise was associated with the onset of the ‘Little Ice Age’, the resumption of war between the Habsburgs and the Turks (1593–1606), and a severe economic recession. At the same time there was a great welling-up of social discontents and political upheavals. The entire frontier zone from Ukraine to the Adriatic was affected by the troubles as well as Russia and the Ottoman Balkans; and there were reverberations in Poland and for the Habsburgs. The crisis was the confluence of many streams and was expressed in many forms, but one of its most frightening manifestations were the bands of undisciplined and ruthless soldiery who plagued both sides of the frontier in Hungary.
The Turks had long used a variety of paramilitary forces (armartolos, derbentsy, akinji, vojnuki, etc.) as auxiliary troops, frontier raiders, mountain-pass guards and the like; as we have seen, the Hapsburgs had followed suit; and the Cossacks constitute a parallel in Ukraine and southern Russia. Such troops usually received some pay and also rations or plots of land, but by no means always. There was an Ottoman category known as deli, young men noted for their dare-devilry who would take part in campaigns and sieges for no reward whatsoever, except the opportunity to share in any plundering. Another such type of predatory soldiery was known as haramia. These had an equivalent on the other side of the frontier in the unpaid heyduks and uskoks (venturini) attached to the ‘official’ groups of heyduks and uskoks employed by the Habsburgs to garrison frontier forts and stations, and the unregistered Cossacks of the Ukraine who were to play such a prominent role in the Khmelnytsky rising of 1648.
Evidence from a wide variety of sources suggests that the numbers of such freelance warriors increased sharply in the later sixteenth century, despite a general increase in the numbers employed not only by governments but in the private armies of noblemen, like the Wisniowieckis in Lithuania, the Bathorys in Transylvania or the Frankopans in Croatia.
This increase in the soldiery, both freelance and employed, and the tumults they promoted were linked to the endemic warfare of the frontier, which created both a demand for such troops and, by disrupting the economy of entire districts, a supply of them from among the ranks of the homeless and indigent. But the phenomenon was also related to the huge increase in the population of the Balkans and to the imposition of serfdom. The demographic explosion which doubled the population of Balkan cities also fed migration northwards and eastwards across the frontier, mostly, it seems, through the gap of Timisoara.
The subsequent economic difficulties and the onset of disorders no doubt increased the flow. In any case the numbers of heyduks called ‘Racz’ registered in Eastern Hungary (and there were units in which nearly two-thirds of the men bore that name) points to a sizeable migration northwards from the Balkans, for racz in Magyar (rat in Romanian) means ‘Serb’. Their names also indicate that, although most were or became linguistic Hungarians, some heyduks had originated in Slovakia (toth), Romania (vlach, olah) and Ukraine (kozak, rusnak) as well as in Hungary and the Balkans. And there were Hungarian, Romanian and Tatar names among the Zaporozh’e Cossacks, though most had migrated from Belorussia, Ukraine and Russia. Circumstances suggest that a proportion of these were peasants escaping serfdom, and this was also the case with the recently enserfed Szekels whose support for Michael ‘the Brave’ when he invaded Transylvania regained them their freedom as frontier servicemen.
As late as the 1580s heyduks are reported in groups of up to a few hundred, or, occasionally, of a thousand; but by the turn of the century no fewer than 8,000 unpaid heyduks were reported to be serving Michael ‘the Brave’, Prince of Wallachia, alone. The growth of the phenomenon is suggested by the extremity of their behaviour as well as increasing numbers. Compared with them, Elizabethan England’s problem with sturdy beggars pales into insignificance. In some areas heyduks claimed to be Calvinist, yet they would kill Calvinist priests without compunction; and the Transylvanian Saxons have left matter-of-fact, but eloquent testimony in their memoirs and diaries to the heartless bestiality of the heyduks.