Economics Zero

Economics is the science that is, among other things, concerned with the allocation of scarce resources.  Ever since the invention of agriculture and civilization, the most central scarce resource has been  agricultural land.  Such land has always been a scarce resource because of the principle discovered by Dr. Malthus - reproduction will tend to exceed the replacement rate and the Earth isn't making more land.  

The dramatic expansion of the feudal system in the centuries just before and after the start of the Second Millennium of the Christian Era owes its dynamism to this fact.  Household knights usually could not marry unless they had a fief of land, and younger sons of a lord also faced the prospect of social status loss.  The solution for such a younger son was to recruit some landless knights and soldiers and set off for a foreign land to rob the local inhabitants.  If such a venture was successful, as in the case of William the Conqueror, the foot soldiers would become knights, the knights would become lords, and the lucky would get fiefs.

William's venture was only the most successful of innumerable such.  By 1300, most of the crowned heads of Europe, and their principal vassals were French - in Spain, Cyprus, Italy, Central Europe, and the Crusader kingdoms of the Middle East.  This vast colonial enterprise transformed Europe.

Humans in much of the world are still breeding at a rate beyond the capacity of the land to support them, and this fact continues to spawn local wars and global floods of refugees.  Meanwhile, the countries that have managed to control their populations are rich enough to mostly avoid these problems, except for the floods of refugees banging at their borders and the occasional intrusion of the various wars on their supply chains.