Question

Once conquered by the Roman army you could become a citizen

Answer 1

Once conquered by the Roman army you could become a citizen "by being a loyal ally for a period of time and joining the army," otherwise it was likely that you might become a slave. 

Answer 2

Once conquered by the Roman army you could become a citizen: native, since the native citizens were those people from other cities who were conquered by the Roman army. When they belonged to Rome they became citizens but they were considered second-class because, because they were not citizens born in Rome, their citizenship had restrictions.