I'm aware they were active in the industry, but I've heard it claimed they were forced into it by medieval kings as, essentially, a way to get someone else to do the "dirty work" for the christian majority. How voluntary was their association with that industry?
In the Tanakh (the Jewish Bible, known to Christians as the ‘Old Testament’), merchants and traders are always Canaanites, never Israelites. Later, in the 1st century CE, Josephus (the noted Jewish-Roman historian) recorded that Jews ‘do not indulge in commerce either by sea or otherwise’. The Jewish Encyclopedia (first published in 1906) also asserts that, in antiquity, ‘not much attention was paid to commerce’ by Jews. The encyclopedia cites, among other evidence, the fact that Jews didn’t mint their own coinage until the Maccabean era (beginning in the 2nd century BCE).
After the Jewish-Roman wars, the Roman conquest of Judea, and the destruction of the Jewish Temple in Jerusalem, Jews began to widely migrate throughout the Roman Empire. By the 5th century CE Jews were well-established in the lands of the Empire, particularly in the Western Roman Empire. In 476 CE the Germanic king Odoacer deposed Emperor Romulus Augustulus, and the Western Roman Empire fell, leaving the Eastern Roman Empire (Byzantium) to take up its mantle.
In Yiddish Civilisation: The Rise and Fall of a Forgotten Nation, Paul Kriwaczek explains that:
Roman trade had been largely under official control, but with the coming of the barbarians, the imperial economic bureaucrats had vanished back to their great hereditary farming estates and cultivated their own gardens, in the process establishing themselves as the new, local, nobility. Jewish smallholders, on the other hand, whose produce was insufficient to pay taxes to the king, whose faith prevented them from dedicating tithes to the local bishop and whose traditions were hostile to providing manpower for war - the three obligations placed on landowners in the new dispensation - faced the difficult prospect of either abandoning their property and retiring to the towns or fleeing overseas, otherwise finding themselves for ever tied to their patch as unfree serfs or villeins in a feudal system that became ever more rigid by the century and which, moreover, insisted on conformity to the Christian religion. It wasn't much of a choice and unsurprisingly most elected to leave the land.
The new kingdoms of Italy, Gaul and Spain, however, desperately needed experienced businessmen. The rough new lords of the West knew little about finance and less of economics, while the arcane art of bookkeeping, which had already been mastered by the Hebrews in Babylonian days (there are clay tablets to prove it from Marashu and Sons' banking and trading house of Nippur, Babylonia, in the fifth century BCE), and without which no commercial concern can successfully operate, must have seemed to beer-quaffing German warriors like so much magic. To them, the Jews of their kingdoms represented the highly valued last remaining survivors of Roman commercial practice.
These events marked the beginning of the European Jewish association with trade and finance. A notable example of a new lord of the West was the 9th century Frankish king Charlemagne.
Charlemagne invited Jews from the Mediterranean to settle in his agricultural lands of Provence and the Rhineland as royal clients, outside of the feudal system, and develop trading communities there. Most pre-WW2 Ashkenazi Jewish communities in Europe had their origins in the Jewish culture, language, and liturgy that began to develop in the Rhineland roughly a thousand years ago.
During Charlemagne’s reign he instituted various financial reforms and did much to standardise currency in Europe. But economic development in medieval Europe was hampered by the fact that merchants typically needed to maintain a large amount of physical currency with which to trade. This was logistically impractical and also economically fragile: currency loses its value as inflation rises, which happened more-or-less constantly during this period. Currency can also be used to purchase assets which maintain their value, such as land. The drawback with the latter is that if one’s capital is tied up in assets, it cannot be quickly liquidated when extra currency is urgently needed. The solution? Investing in loans, and charging interest to compensate for the currency’s loss of value over time, ie. proto-banking.
Christian doctrine forbade lending money to fellow Christians and charging interest. But Jews could lend money to Christians and charge interest. This system was of vital importance for economic growth and development in Europe, and moneylending became increasingly indispensable, not just for monarchs but also nobles, merchants, and even peasants.
The Jewish religion is inherently community-oriented, requiring a minyan (a quorum of ten adult men) for many important prayers, and a need for locally-available kosher food and wine. This, combined with the widespread prohibition against Jewish land-ownership in medieval Europe, led the majority of Jews to settle in urban areas. Trade guilds began to emerge in Europe in the Middle Ages, from which Jews were always excluded, thus forcing them out of the skilled and artisanal trades and crafts popular in cities and towns. Consequently Jews became vastly overrepresented in European commerce and moneylending, they had little choice.
Following the Norman Conquest of 1066, William the Conqueror invited Jewish merchants from Normandy to settle in England, believing that their commercial knowledge and skill would help increase England’s prosperity. Later, under King Henry I (as in many kingdoms of mainland Europe at the time) the Jews of England were considered the property of the Crown.
English and other European kings frequently exploited Jews financially, using them as a source of income. For example, In England the office of Exchequer of the Jews was created, following the 1194 Ordnance of the Jewry. The Exchequer taxed all Jewish transactions on behalf of the King of England, essentially making the king a ‘sleeping partner’ in Jewish moneylending and trade. Furthermore, if the king’s courts and officers aided Jews in recovering debts, the king was entitled to ten percent of the sum recovered. Ever-increasing anti-Jewish violence (massacres, blood libels) and legislation (requiring Jews to wear badges, further monetary exploitation) in England ultimately culminated in the 1290 Edict of Expulsion, which expelled all Jews from England, seized their property, and ruled that all outstanding debts to Jews should be paid to the king. Jews were not able to return to England until the mid 17th century, under Oliver Cromwell.
Countless books and dissertations have been written on this subject but, in short, Jewish involvement in finance and commerce was almost entirely involuntary. It developed over a long period of time and for manifold reasons related to their vulnerable minority status since the exile from Judea. Despite this fact, Jews became symbols of money and exploitation, and were frequently used as scapegoats by monarchs, nobility, and moneyed landowners to direct anger from serfs and peasants away from themselves. This ultimately resulted in Karl Marx (himself a Jew, albeit one remarkably ignorant about Jewish history) writing his antisemitic 1843 essay On the Jewish Question, in which he stated that ‘The bill of exchange is the real god of the Jew’.