Let's clarify that for a serf, support meant military supply, not swinging a sword - that was reserved for the knightly class. For the great majority of medieval villagers the tie to their lord revolved around getting crops out of the ground.

A serf's week was scheduled around the days they worked the land whose proceeds went to the lord and the commons that subsisted themselves. Transfers of grain and livestock from serf to lord along with small dues in eggs, wool, or coin primarily constituted one side of the economic relation between serf and lord. These transfers kept the lord's demesne barns full so he could sustain his household, supply retainers, etc, not to mention fulfill the. tithe that sustained the parish.

While peasants occasionally marched, they contributed primary in financing war more than they fought it. Their grain, rents, and fees were funneled into supporting horses, mail, crossbows rather than being called to fight themselves.