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The concept of praying to the Roman Gods as well as to whatever local deity did mean that the Republic then Empire could assimilate a lot of cultures. After all, they were always worshuiping the same gods, and now they can have access to all the good things that Rome provides -- see Life of Brian's "What did the Romans ever do for us?" speech. Even when the Empire shifted to being a Christian one and imposed one religion, most of the Barbarian tribes wanted in because they could see advantages to being in the Empire as opposed to out.
There was no official assimilation but first roads would be built to allow the Legions to get places faster. This would reduce tribal warfare, increase commerce, and facilitate movements of goods/people. Once this happened, immigration would start, the locals would start to want the same standard of living as Rome and thus Romanisation would happen. Other places, it was a military conquest, with troops on the streets and martial law. Some of those places would fall into peace others would not.
Scipio Africanus has a general method of expending the republic (Iberia, Numedia, Carthage) which involved making friends with everyone he met. Even his terms toward Carthage at the end of the war were remarkably generous to the dismay of may in the Senate. Caesar went into Gaul to answer the call of help of tribes there. He was invited -- or so he claims -- because some of the Gaul wanted to join Rome.
Source: Scipio Africanus: Greater Than Napoleon B. H. Liddell Hart. Caesar: The Life of a Colossus by Adrian Goldsworthy.
Upvote:3
They annexed territories for tax generating and mercantile trading, food generation purposes and integrated some citizens for service in the legions.
It's interesting that later on Rome's policy of allowing mass immigration and citizenship for many people from the annexed territories is one of the many reasons for the Empires ultimate demise.
Large scale immigration of this scale if managed correctly can have positive beneficial effects on economies, if mismanaged can have detrimental effects. This is something I believe modern societies can learn from - history has a habit of repeating itself and we should be able to learn from past mistakes.
Upvote:4
I am not sure this question would have made much sense to the Romans themselves. The whole concept of formal guidelines/directives is something which is associated with the modern bureaucratic state (especially in its totalitarian forms). The Romans just didn't work that way and probably would have been amazed at the suggestion that the Senate or the Emperor needs to formally delineate such procedures. There were no procedures.
Also, they were usually very tolerant of other cults, often adopting them themselves eventually (over a number of generations; the cult of Isis is just one well-attested example).
ADDITION: However, the Roman state did have policies in place that enabled conquered peoples to be assimilated into the Roman polity. The broad principle was that it took a few generations and some effort. The wikipedia article on Latin Rights is a decent quick summary.