It's a mistake to think Romans had a policy of assimilation. For the most part they didn't care what people in the provinces did as long as they played along. There were some mechanisms that had the inadvertent effect of "Romanizing" people (and that word is terribly problematic; it's not as if cultural influence is a one way street).

Auxiliaries in the armies had to know Latin; access to the Mediterranean economy spread the consumption of goods and ideas not available elsewhere. The need to have access to the center of power (Rome) encouraged local elites to develop relationships with Elites in Rome (The Balbi family's connection with Caesar is a good example).

On the other hand a lot of native forms continued. Carthage was rebuilt using local construction methods; while Britain developed a villa-based agricultural system, it looked very different from the Roman sort. And of course where there was already a common language and integrated economy (the Greek East) Romanization didn't have much effect at all.

You may be interested in: Revell. Roman Imperialism and Local Identities. Cambridge University Press, 2008.

Keay and Terranato, eds. Italy and the West. Oxford: Oxbow, 2001.

Hmm, I don't really see much truth in that quote. There were a few cases in which the Romans engaged in mass deportation, most famously with Carthage and Jerusalem, but those were always in fairly limited and specific areas. I don't really see that being feasible in a larger area--imagine the logistics of deporting, say, the population of Gaul.

That seems to be a weird offshoot of the early theories of a colonial process of Romanization. I am most familiar with Britain, where before the development of systematic archaeology it was always assumed that Roman Britain was essentially a military base, with forts and the city of London, with the remainder largely continuing pre-Roman lifestyles. With the development of archaeology in the nineteenth century, however, people starting digging up mosaics, Roman style walls, Roman pottery, and looking to their own experiences of Empire the general assumption was that these villas and towns were inhabited by Roman colonists. For whatever reason, popular opinion has seemed to vacillate between these two narratives: either Roman Britain was a bunch of forts, or it was a bunch of forts and colonies. In 1914, however, a scholar named Francis Haverfield published The Romanization of Britain, and although it is explicitly a cheerleader piece for Empire ("Our culture to-day seems firmly planted in three continents and our task is rather to diffuse it further and to develop its good qualities than to defend it. But the Roman Empire was the civilized world; the safety of Rome was the safety of all civilization. Outside was the wild chaos of barbarism.") its basic assumption, that the Roman material in Britain is not from colonists but because of an internal transformation of the inhabitants of Britain towards a Roman style of life, has held. There were undoubtedly colonists (or immigrants), but the vast majority of change you see is an internal process among the Britons themselves.

Now, hashing out the details and meaning of this transformation is something that has been going on for a very long time and is probably never going to stop. Was it an imposed and conscious cultural policy on the part of the Roman authorities? Was it a process of imitation by the elites that trickled down to the populace as a whole? Was it an economic process? Was it due to the inherent appeal of Roman goods, or imperialist social structures? Was the material transformation a thin veneer masking essentially continuity? Were the inhabitants Romanized? Creolized? Did they become Roman? What does it even mean to be Roman or British?

This is one of the essential questions of the Roman Empire, and the debate tends to be more centered around the approaches and predilections of the various scholars that alternate data sets. My personal favorite book on the subject is Greg Woolf's Becoming Roman, but the bibliography is vast and unending.

Its true to a degree, the Roman legions were often garrisoned in conquered places to ensure stability (legions being only men). This did cause some interbreeding between roman legionnaires and locals. It also caused trade between the locals and the legions, as roman coin and luxuries were highly valued by the locals. While the legions needed food and other essentials, which were cheaper to buy locally than from other regions of the empire. This trade did turn the border into more of a gradient than a proper border, as the legions had influence outside of the roman empire through trade and vice versa. Which has been argued to be one of the reasons for the Western Roman Empires fall. As the stability of some regions was based on the locals wanting roman coin and luxuries. When these lost their value for the locals, the legions which had just as big an allegiance to the locals after years of trade and interbreeding as to Rome. Caused the legions now as Germanic groups and Franks to turn onto the empire as their allegiance no longer lay in Rome.

Sources: http://www.worldcat.org/title/roman-europe-1000-bc-ad-400/oclc/300297548&referer=brief_results http://www.worldcat.org/title/europa-tar-form-ar-300-til-1300/oclc/214764449&referer=brief_results

Prior to Christianity, the Roman Republic used religion to their advantage. Taken from Wikipedia

As the Romans extended their dominance throughout the Mediterranean world, their policy in general was to absorb the deities and cults of other peoples rather than try to eradicate them

As my Roman Republic professor said, the Romans would go to the extent to take statues of conquered deities and bring them back to Rome to be worshiped. This in most cases would eliminate holy wars against Rome, as it is difficult to declare a holy war against the city that is worshiping the same god as you.

In this sense it was less Rome forcing assimilation, rather Rome would assimilate to the areas it conquered