The Crusades, a series of religiously motivated military campaigns launched by western European Christians beginning at the end of the eleventh century, were framed and fought as wars over control of Jerusalem and the wider Holy Land, sanctioned by the papacy as a religious duty. Their stated goal was overwhelmingly religious. Their most durable historical legacy, though, had less to do with who controlled Jerusalem — which changed hands multiple times over the campaigns and eventually settled back under Muslim control — than with a byproduct nobody had set out to achieve: renewed, sustained contact between western Europe and the eastern Mediterranean world, at a scale that had a lasting economic and intellectual effect on Europe long after the military campaigns themselves had ended.
A religious campaign that opened a commercial corridor
Sustaining crusading armies and the flow of pilgrims and settlers to the Crusader states established in the Levant required regular shipping and supply routes across the Mediterranean, routes that Italian maritime republics like Venice and Genoa were well positioned to run, and did so extensively and profitably. That sustained shipping traffic, originally organised to support a religious and military campaign, functioned in practice as a durable commercial corridor, reintroducing western Europe to a volume and range of eastern Mediterranean and Asian goods — spices, silk, dyes, and other luxury goods — well beyond what had reached most of Europe in the preceding centuries. Italian merchant cities that built their shipping and trading infrastructure around supplying the Crusades went on to become major, wealthy commercial powers in their own right, an economic transformation the crusading movement never set out to produce but substantially enabled.
Contact that carried more than goods
Alongside goods, the sustained contact the Crusades produced between western Europe and the more urbanised, in several respects more scientifically and intellectually advanced societies of the eastern Mediterranean and Islamic world brought increased exposure to preserved and extended classical Greek scholarship, along with advances in mathematics, medicine, and other fields that had continued developing in the Islamic world during a period when access to that same classical scholarship had been comparatively limited in much of western Europe. Historians generally treat this increased contact as one contributing factor, among several, in the broader flow of knowledge back into western Europe that fed into later intellectual developments — though, importantly, only one contributing factor among several, since trade and scholarly contact along other routes, and the preservation and transmission work already ongoing within Islamic scholarly institutions independent of the Crusades, were also significant and shouldn't be credited to the Crusades themselves.
What we're still unsure about
That the Crusades produced substantial, lasting commercial and cultural contact between western Europe and the eastern Mediterranean, benefiting Italian maritime republics in particular, is well documented and widely accepted among historians. What remains genuinely debated is exactly how much causal weight to assign the Crusades specifically, relative to other channels of contact and exchange that were also operating during the same broad period — trade through Iberia, Sicily, and other routes not directly tied to the crusading campaigns — and historians differ on how central a role the Crusades themselves deserve in the larger story of medieval Europe's economic and intellectual development, as opposed to being one visible episode within a broader pattern of contact that would likely have continued developing along other channels regardless.
This sits inside The Crusades, one of eight topics in Medieval History, one of seven domains in History, one of seventeen subjects the app can quiz you on.