"Or go the way of Ancient Rome, abandon military aspirations as expensive and frivolous, liberalise and focus on developing our society and its artistic nature?"
Boy, you really don't know ANY history, do you? Where did you get your facts about ancient Rome - Hollywood?
Briefly, the Roman Empire gradually left behind the features that had allowed the Roman Republic to conquer most of its known world. Government was more and more centralized, until a single man - the emperor - was effectively in control of everything that happened. Meanwhile, economic changes saw bankers and the rich steadily hoover up more and more of society's total wealth. The super-rich bought immense estates (latifundia) worked by super-cheap slave labour, while the free small farmers who had previously been the backbone of Rome (and the main source of its citizen armies) disappeared. The Roman world become more and more sharply polarized between the vastly rich and the grindingly poor - whether slaves or impoverished share-croppers most of whom were actually worse off than the slaves. Even in towns and cities the same thing happened. In frantic efforts to raise taxes still higher, the government eventually forbade all social mobility, ordering sons to follow in their fathers' business and stay in their place of birth.
With the destruction of the free smallholders and the middle class, it was no longer possible to raise free citizen armies. Instead, the emperors resorted to paid mercenaries. As they had no loyalty except to whoever was paying them, those soldiers became a dangerous political force - often murdering unsuccessful generals and even emperors. As the surrounding barbarian tribes closed in, the emperors had to find more and more colossal sums with which to buy them off (and to pay the mercenaries as a way of bluffing the barbarians that the cities could actually be defended). And that meant the taxes had to be increased still more.
Finally, the barbarians became the mercenaries, so that when they revolted and killed an emperor it was natural for them to replace him with one of their own chiefs. That's about when Rome actually fell. (A similar process later reduced the Byzantine Empire to a tiny remnant). But Roman civilization had already dissolved - slowly, imperceptibly - over hundreds of years. If you have read this comment carefully, you may have noticed a few trends that are reminiscent of our world today. An ever bigger and more centralized government; monomaniacal focus on military improvements; and the gradual takeover of society by the super-rich. It's all playing out again.