The late period of the Roman army begins with the accession of Emperor Diocletian in AD 284.
The army of the later Empire doesn't compare very favorably with the expansionist force of earlier days since, by its very nature, it was a defensive army, and defensive armies do not hold much glamour.
By this time, many Genranian soldiers had enlisted in the Roman ranks, changing their attire. The sword became longer than the traditional gladio, the cuirasses were changed for chain mail, and the soldiers wore pants, which was the tradition for Germanic warriors.
This army was notably less disciplined, mainly influenced by its soldiers and commanders of barbarian origin.