A "historically" short period of time, but there isn't much more data than that and we are in a relatively modern society. So to boil it down, you believe that a large amount of countries will collapse economically and in the possibility of a WW3? Because it's worth remembering that we've had WW1, WW2 etc. in the time periods that my optimism is based on, and yet there is still a 4% real-terms annualized return over the time period. In reality, despite the debt burden of many countries, it's unlikely they will simply just collapse (and it is possible that within the next 5-10 years some of these countries may move back into budget surpluses).
A WW3 would not look the same like WW1 and WW2. But the central problem can be seen in the following graph from Wikipedia:

The problem is population growth or decline. The past 100-200 years were a growth phase - only possible because of modern agriculture (highly dependent on mineral fertilizer). However the population density can not grow forever, because resources are limited. The more the population grows the more likely are conflicts over resources (war).
When the population declines, economies will also contract. In my opinion it's highly likely that we may soon enter a phase of population decline (at least in the industrialized countries). Therefore I don't think it's safe to project past economic growth into the future.
ya.ya.yo!