Different kinds of armies and a consideration of mercenary forces
Any state—old, new, whatever—needs good laws and good armed forces.
Since you can't have good armed forces without good law, let's just say you need a good army. There are four types of armies you could have: a local army, mercenaries, auxiliaries (helpers from someone else's army), or some kind of mixture.
First things first: mercenaries and auxiliary armies are useless. Just don't do it. Mercenaries are only interested in the money and are not reliable. That's how Italy got into trouble—occupation by France and Spain—in the first place.
Plus, if a mercenary leader is good then you have to be afraid that he will turn against you, and if he is bad he will make you lose anyway. So, no good.
Good armies? Citizen armies. Look at Rome. Look at Sparta! Carthage used mercenaries, and guess what happened to them? They got owned by Philip of Macedonia, Alexander the Great's dad. Don't be like Carthage.
Machiavelli gives us a bunch of examples, but the basic takeaway here is mercenaries = bad. They are lazy. They are expensive. They kill during wartime. And they don't even defend their camps.
Machiavelli has a little bit of an axe to grind about this problem, since he has wanted Italy to stop relying on mercenaries forever, but no one would listen to him.