We're going to have to disagree there, on at least three points. Many countries which have liberal values today still practice compulsory national service - I can think of two just off the top of my head.
Your statement is also incorrect in it's implication that the state will simply back down from achieving it's political/strategic aims if it fails to raise sufficient troops through volunteerism alone. History has repeatedly shown this to not be the case - Britain in WW1, Australia in WW2, Australia again in Vietnam, the U.S.A. in Vietnam are three blatantly obvious examples (indeed in the latter, the U.S. Government never officially called out the Reserves, but did impose the Draft).
Finally, your statement rests on the demonstrably untenable notion that soldiers who can volunteer do so with financial inducements as their prime objective. This is practically incorrect in my case, I think practically in MobBoss' case, as well as being unable to be theoretically universally applicable as an absolute statement because the military is not "just for the poor" in all countries even if it is in the U.S. (indeed, in my case the last ADF Defence Census actually showed a vast majority of respondents hailing from Middle or Upper Class Australia, as Lower or Working Class Australians weren't able to meet the entry qualifications in the same numbers).... I think I just demonstrated this notion false using three different examples.
Sorry mate, you'll need to do better than that.