There was quite a bit of emphasis on serving in the military, and as I recall, in the Republic days, military service was required to have a political career. And one of the reasons for the continual conquest was that this was a way to make the masses happy, because it brought riches to their country from these conquered lands, and brought political opportunity for the patrician class because of the need to administer these new provinces. In any case, discipline within the army was brutal, with decimation being the classic example, where the soldiers were separated into groups of ten and forced to kill one of their number, or they would all be killed.ChiggyvonRichthofen wrote: As far as Rome goes, I'm no expert, but I would credit its long-term survival to its stability, orderliness, relative political and economic sophistication, and simply being too large and expansive for smaller nations to have a hope to successfully invade. Assassinations, political upheaval, and cruelty were survived (and all things that were pretty common everywhere, not just with Rome), but those things played their role in Rome's eventual decline as well.
Later, during the waning days of the Empire, military service didn't have the same kind of honored distinction anymore, and this is where the famous "mark of the legion" came in so that deserters could be found more easily. Rome came to depend more and more on the Auxiliaries for their defense (which were largely made up of foreigners who were trying to earn citizenship), and politically pretty much everything was focused inward.