It's probably a matter of degree. The crappier your fields are, the more you have to rely on herding and hunting, and there's a tendency for this to lead to a more warlike nature (if for no other reason than that you're out of options otherwise). The British in India classified some tribes, mostly hill-based ones, as "martial", seeing them as better recruiting grounds for local soldiers due to a stronger martial ethos.
In Glorantha though, this also has an actual impact - if you're a War clan, you focus more on the Death rune than the Life rune, which will make you better fighters and worse farmers. Since so much of clan warfare is raiding, even a larger Peace clan may have troubles bringing its numbers to bear, and paying a reasonable tribute (which they can afford, being better farmers) will make sense for both sides.