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During the reign of Augustus the Rome army became a professional one. Its core of legionaires was composed of Roman citizens who served for a minimum of twenty five years. Augustus in his reign tried to eliminate the loyalty of the legions to the generals who commanded them, forcing them to take an oath of allegiance directly to him. While the legions remained relatively loyal to Augustus during his reign, under others, especially the more corrupt emperors or those who unwisely treated the military poorly, the legions often took power into their own hands. Legions continued to move farther and farther to the outskirts of society, especially in the later periods of the empire as the majority of legionaires no longer came from Italy, and were instead born in the provinces. The loyalty the legions felt to their emperor only degraded more with time, and lead in the 2nd Centry and 3rd Century to a large number of military usurpers and civil wars. By the time of the military officer emperors that characterized the period following the Crisis of the Third Century the Roman army was just as likely to be attacking itself as an outside invader.
Both the pre- and post-Marian armies were greatly assisted by auxiliary troops. A typical Roman legion was accompanied by a matching auxiliary legion. In the pre-Marian army these auxiliary troops were Italians, and often Latins, from cities near Rome. The post-Marian army incorporated these Italian soldiers into its standard legions (as all Italians were Roman citizens after the Social War). Its auxiliary troops were made up of foreigners from provinces distant to Rome, who gained Roman citizenship after completing their twenty five years of service. This system of foreign auxiliaries allowed the post-Marian army to strengthen traditional weak points of the Roman system, such as light missile troops and cavalry, with foreign specialists, especially as the richer classes took less and less part of military affairs and the Roman army lost much of its domestic calvary.
At the beginning of the Imperial period the number of legions was 60, which Augustus more than halved to 28, numbering at approximately 160,000 men. As more territory was conquered throughout the Imperial period, this fluctuated into the mid-thirties. At the same time, at the beginning of the Imperial period the foreign auxiliaries made up a rather small portion of the military, but continued to rise, so that by the end of the period of the Five Good Emperors they probably equalled the legionaires in number, giving a combined total of between 300,000 and 400,000 men in the Army.
The last major reform of the Imperial Army came under the reign of Diocletian in the late 3rd Century. During the instability that had marked most of that century, the army had fallen in number and lost much of its ability to effectively police and defend the empire. He quickly recruited a large number of men, increasing the number of legionaires from between 150,000-200,000 to 350,000-400,000, effectively doubling the number in a case of quantity over quality.
In the post-Marian army,______