If survival of the fittest is truly natural then I can say this:
War is a natural way of getting rid of competition in a communal/ tribal/ colonial/ empirical/ national/ global/ galaxial/ universal level.
When Darwin stated "survival of the fittest", it was meant the one most able adapt to change. Change is what causes issues. Adjusting to change will lead to a more stable nation, rather than having the largest military.
For example, China has the largest conventional military by sheer solider count. However, at a time of war, when resources are very limited, how well will the country fair? Trying to feed 300 million soldiers at war? Good luck.