The answer to this question is a matter of debate for historians, and really would depend on the time period. The ability of Europeans to expand and conquer peoples around the world was unquestionably related to their relative strength, particularly in terms of technology. But their desire to do so was largely related (at least at first) to their perceived economic weakness compared to the Muslim world. European merchants and monarchs wanted to force their way into the lucrative gold (Africa) and spice (Asia) trades that were controlled, at least in the Western world, by Asia. So it could be argued that Europeans were driven by their own sense of weakness. In the nineteenth century, many argued that taking colonies around the world was a means of securing captive markets, which were seen as essential to continuing economic growth. So this also could be seen as a motive related to Europeans' belief in the tenuousness of capitalist expansion.