I don't think it's controversial to say that there's way more going on behind the effort to staff an army than threat of punishment from the government. Historically there are many examples of citizens eager to line up and fight for their country (whether for pragmatic or idealistic reasons). This isn't just patriotic rhetoric, it's a very powerful social force once set in motion.

If our species has any enduring trait then violent conflict through war must be pretty close to the top of the list.