My understanding is that during the Roman Empire's expansion, many villages and tribes were glad to join Rome, because men who went into the Roman military were payed very well, and their families prospered, under the ability to go to Rome and get an education. Which is what many of the village men's sons did.
That was assimilation, social and military requirements. Before the Marian Reforms Rome's armies were based on social class thus wealth. Soldiers bought their own equipment which dictates their position in the legions. The poor were only used in emergencies. As Rome expanded this system could no longer supply the manpower and quality of arms to hold 2nd to 1st century BCE Republic territory. Marius removed the wealth requirement and provided standardized gear by the state. These are the iconic Legions of Rome. Social status dictate who was a soldier and who was a mercenary. Marius remove the social requirement thus anyone could join the Legions. Mercenaries were those that never signed a contract typically cavalry. Terms of service was 25 years. After service was complete the legionnaire was granted Roman citizenship which put that solider above Italians, freemen, foreigners, slaves, etc, everyone that was not Roman. Roman citizenship was to be part of the elite even if one didn't have a lot of money. The status alone opened a lot of door typically in politics. At that point the Legions were professional armies. Pay, and looting rights, was high due to be state sponsored instead of based on loyalty/patronage bonds. It was one of the best paying jobs an average able-bodied male could find.
Flash forward a few centuries the Romans were destroyed by tribes that knew Roman tactics, fought in Roman armies, were leaders of those armies. No bonds of loyalty between various tribes and Rome last long. The same people were willing to destroy anything just to get the prize of Rome even if they destroyed it along the way. The legions became loyal to the generals not the state. The system backfired.
Colonization is a dead method. Humans filled to many areas of the world now. Colonization in my view is only to be used in relation to interplanetary travel and the solar system. Immigration works fine on Earth. It provided the ME with the technology, training, expertise, etc for it's oil industry.