Perhaps the single biggest factor behind the exploration by Western European nations in the Middle Ages was trade. These voyages abroad by Western Europeans took on the greatest significance in the late Middle Ages, particularly in the fifteenth century. Voyages motivated by commerce were of course nothing new, either in Europe or outside of it, but in the period under discussion there was a more particular reason for the Western European nations - principally Spain and Portugal -to undertake their great sea voyages which helped open up new global perspectives. In the main, the Spanish and Portuguese were endeavouring to find new routes to old commodities such as silk and spices, to discover new ways of getting to these lucrative centres of trade in Asia and the Far East. They attempted this by sailing west, which among other things led to the discovery of the Americas (Columbus) and new landfalls on the Indian coast (Vasco da Gama).
Along with this commercial motive the Europeans were also actuated by missionary zeal, and generally they sought to Christianize any natives of other lands that they came across. Because of this, these great voyages of exploration have sometimes been characterized as a continuation of the Crusades in another form.
There were several important developments that facilitated western European expansion at this time. One was the general political situation. After a prolonged period of incessant wars and disasters such as the Black Death, western Europe was beginning to gain a bit more stability. Spain, for instance, had recently become unified by bringing together the great royal houses of Aragon and Castile, and had expelled the last of the Moors, and was in a position of new strength and confidence which encouraged the development of imperial ambitions abroad.
Crucially, there were also significant technological developments in this period: advances in sea-faring implements such as charts, compasses and, most of all, bigger and better ships. All of these played a vital role in shaping the first great wave of European expansion abroad.