Conquest and Culture
Arun and I have been "discussing" the role of conquest and colony in cultural spread. In today's world of movies, television, and the internet, there is certainly a fair amount of cultural spread by means less direct than conquest, and there was some even in the distant past, but the evidence is strong that conquest was the big driver, especially of linguistic change. Languages are hard. The big linguistic changes that we have seen in history (Latin, Arabic, English, and Spanish, for example) were driven purely by conquest. There is good reason to suspect the same of prehistoric language expansions.
Such linguistic expansions have brought vast cultural shifts in their wake, including massive changes in agriculture, economic organization, and political institutions. None of which proves that colonialism is good, or even had good side effects. But it does mean that essentially all of our cultures have been strongly shaped by those conquerors.
One of the huge mysteries of the prehistoric past is that of the origin and spread of Indo-European languages and culture. It's probably not important whether you accept one of the standard theories (origin in central Asia or Anatolia) or one of the fringe theories, like the Out of India theory, there is very good reason to suspect that conquest was the mechanism of spread. You might voluntarily copy a foreigner's technology, but you don't adopt his language unless there is overwhelming necessity to do so.