Essentially, I am imagining this scenario:
Somewhere there is a world where, in all the major civilizations, assassins are more numerous and more highly respected than soldiers. In such a world, wars as we know them virtually never happen, but attempts by high-ranking figures of opposing groups to have each other assassinated would be so common that they would have just as many assassins as we have soldiers. This would also carry over to how soldiers and assassins are perceived by the people of this world; the people would view assassins as fulfilling a regrettably unavoidable but highly honorable service to their nation, much as we view soldiers, while they would view soldiers, even those who operated in the strictest approval of their own government, with as much suspicion as we view assassins.
My question then is: How could this scenario happen?
I am imagining a scenario where this has been the case since prehistory, so technological reasons are out. I would prefer answers dealing with an effectively human society, so scenarios essentially featuring normal humans dropped into another world at the beginning of history are ideal, but it would also be perfectly acceptable to give a reason why the biology of a certain kind of sapient species would lead it to a preference for large-scale institutions of assassination over militaries as humans know them.