Veterans may have retired (different to resigning), or been demobilised (like being made redundant).
You can't resign from conscription, some countries have a system whereby conscripts could serve non-combat roles; but usually it's fight or death/imprisonment.
Apparently Germany had general conscription from 1935, but with lots of exceptions. By 1943 all men up to 60 were being conscripted. Follow orders or face a firing squad.
It doesn't, it's background on a side-thread response to "just following orders". The reference to WWII Germany being because the usual origin is that this was used as a mitigation by those involved in the Wehrmacht.