Feudalism was the dominant social system In Europe for much of the Middle Ages but began to decline in the late 1400s. Historians have identified a number of reasons for this decline, beginning first with the Black Death, the plague which ravaged Europe between 1348 and 1352. This devastating epidemic caused the deaths of approximately one-third of medieval society, from the lowly serf to the landed gentry. When the plague had passed, peasants demanded higher wages and better conditions and their social superiors had no choice but to give in.
Another important reason for the decline of feudalism was the growing power of medieval kings. The Hundred Years War (1337-1453) between England and France, for example, showed that monarchs could consolidate their power and boost their prestige without the help of their noblemen, society's feudal lords. This lessened the feudal importance of the nobility and paved the way for absolute monarchy - a system in which feudalism was unnecessary.
Finally, trade had never been compatible with feudalism. It encouraged peasants to be economically active outside of their manor - by trading with other peasants and building roads, for example, both of which were completely forbidden under feudalism. As the Middle Ages went on, countless opportunities for international trade blossomed and this threatened feudalism to its very core. The Crusades, for example, facilitated contact between the Christian West and Islamic East. The Italian city states were, arguably, the biggest benefactors of these trading opportunities and as their wealth increased, they demanded an end to their oppression - and the rest of Europe soon followed suit.