The Constitutio Antoniniana (Latin: "Constitution [or Edict] of Antoninus") (also called the Edict of Caracalla) was an edict issued in 212 , by the Roman Emperor Caracalla declaring that all free men in the Roman Empire were to be given full Roman citizenship and all free women in Empire were given the same rights as Roman women.
Before 212, for the most part only inhabitants of Italia held full Roman citizenship. Colonies of Romans established in other provinces, Romans (or their descendants) living in provinces, the inhabitants of various cities throughout the Empire, and small numbers of local nobles (such as kings of client countries) held full citizenship also. Provincials, on the other hand, were usually non-citizens, although some held the Latin Right.
According to Cassius Dio, the only Roman historian to talk about this edict, and with only one sentence, the reasons Caracalla passed this law were mainly to increase the number of people available to tax. In the words of Cassius Dio: "This was the reason why he made all the people in his empire Roman citizens; nominally he was honouring them, but his real purpose was to increase his revenues by this means, inasmuch as aliens did not have to pay most of these taxes." It should, however, be noted that Cassius Dio generally saw Caracalla as a bad, contemptible emperor.
Another goal may have been to increase the number of men able to serve in the legions, as only full citizens could serve as legionaries in the Roman Army. Despite what might have been seen as easy benefits, the edict came at the cost to the auxiliaries, which primarily consisted of non-citizen men, and led to barbarization of the Roman military.
Additionally, before the edict, one of the main ways to acquire Roman citizenship was to enlist in the army, the completion of service in which would give the citizenship to the discharged soldier. The edict of 212 may have made enlistment in the army less attractive to most, hence the recruiting difficulties of the Roman army by the end of the 3rd century.