The unlimited _imperium_ of the consul in the field (_militiae_), which was asserted when he had crossed the _pomerium_[902] and required the sanction of the _lex curiata_, was, in the early Republic when wars were confined to Italy, generally exercised by both the consuls together. To avoid the inconvenience and danger attending the rule of two commanders-in-chief of equal power, the principle of rotation was adopted, each consul having the supreme command for a single day.[903] But this device was necessary only when military considerations dictated that all the Roman forces should act together. Frequently the Roman armies had been simultaneously directed against various points of Italy, and the custom naturally suggested itself that each consul should command half of the regular army of four legions, and thus have an independent sphere of operations (_provincia_).[904] In a defensive war, such as that against Hannibal, Italy would naturally fall into two consular provinces;[905] but the practice became even more essential when the Roman arms extended beyond the peninsula, and in the period of the acquisition of the empire, from the beginning of the first Punic war to the close of the struggle with Greece (264-146 B.C.), _Italia_ as a whole, and some foreign country such as Greece or Macedon, are the regular _provinciae_ held by the consuls.[906] The arrangements which were made for the permanent government of provinces, first through praetors and afterwards through pro-magistrates, tended to arrest their employment for this purpose; but down to the time of Sulla (81 B.C.) a consul might at any time be appointed to a transmarine province.[907]
The consuls settled the distribution of _provinciae_ by agreement or by lot,[908] the _sortitio_ becoming in time the more usual practice. Occasionally the Senate ventured to suggest that one of the consuls was better qualified for a special department, and in this case the inevitable consent of his colleague enabled him to assume it _extra sortem_.[909] But, as Rome’s activity extended, and the available magistrates with _imperium_ increased, the important question came to be, not who should have one of two departments, but which should be the consular provinces. This power to nominate the provinces (_nominare provincias_) had, by the close of the Hannibalic war, become the undisputed prerogative of the Senate,[910] and one of its surest modes of controlling the consuls. This _de facto_ power was formally recognised by a law of the tribune C. Gracchus in 123 B.C., although it scarcely required legal recognition, and the purport of the _lex Sempronia_ was to weaken the discretionary power of the Senate by enacting that the consular provinces should be fixed before the election of the consuls who were to hold them.[911] At this period the consular departments were almost invariably foreign commands; but, after the close of the social war and the reforms of Sulla, they were held by their recipients as proconsuls after their year of office at Rome had expired.