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4th-3rd Century BCE
Establishment of the early legion and abandonment of the Greek phalanx. Often attributed to the Samnite Wars (343-290 BCE).
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340 BCE
Roman historian Livy accounts for a legion of 4,000-5,000 men. The manipular legion was organized into three lines of 15 maniples each.
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Early 3rd century BCE
Greek historian describes the legion by the Second Punic War (221-202 BCE) as three lines of 10 maniples each. Soldiers were equipped with an oval shield, gladius, and either a javelin or spear.
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107 BCE
Gaius Marius reformed the legion, replacing the maniples with cohorts in 107 BCE. Soldiers were equipped with a gladius, pugio, and pilum, and armored with hamata mail.
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1st Century CE
Known as the "classical legion". Augustus (r. 27 BCE - 14 CE) creates 28 legions of 6,000 men each. Changed the length of service to 20 years. Establishment of the aquilifer and aquila.
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250 CE
The use of the gladius and pilum starts to decrease around this time. Border warfare caused the legions to become disorganized and forced the recruitment of barbarians to fill auxiliary ranks.
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4th-5th Century CE
The Roman army faced changes that required an increase in cavalry and decrease in infantry. Emperor Theodosius (r. 379-395 CE) began to heavily recruit German horsemen. The legionary cavalry was essentially replaced by the heavier Germanic cavalry.