well, between the years 404 c.e. (some versions have 476 c.e.) and 1453 c.e. europe is considered to have been in a dark age of sorts. it is an arrogant way of saying there was little technical as compared to the days of the roman empire. in that time it appears the eastern world, mainly dominated by muslim countries (also inheritors of the eastern roman law) and in the far, china and japan, was very difficult terrain for the europeans to maneuver.
sometime during the late 1300's gunpowder became more widely seen as a useful weapon of war. over the next century european engineers perfected it for battle and by 1453, they were able to defeat the greatest of the eastern powers, the Ottoman empire (modern day turkey).
Over the next 50 years, Europe began to rely less on the old religious fanaticism of the dark ages, then the idea that the earth had much to offer their humanly (i.e. greedy) needs so, in addition to science, art, and philosophy, the need to explore and conquer became popular. There were still fables from the 1200's-1400's which were widely believed of lands far away with gold and unbelievable wealth, as well as mystical creatures, such as brought to Europe by marco polo, legends of prester john whom was a mythical ruler of ethiopia or possibly ghengis khan as others have surmised.
To make it simple, by 1500, they had the technology, the weapons, and the desire to go out and see and conquer just that they lacked accurate information.
hope that sorta helps.
