Historically it was extremely hard to "reuse" former troops. 16th century warfare aged men fast between disease, privation, and wounds. And as males got older they married, had families, and were very reluctant to leave the farms. Also men who had served once got very good at disappearing when recruiters showed up in the village. They had a pretty good idea what to expect and wanted no part of it. It wasn't until much later that standing militias and professional armies allowed the call up of "reservists".