And changed at least twice between Caesar and Justinian. Furthermore the equipment also went down hill in that time period. This was in addition to a decline in pay. There were no pay increase from the time of Augustus till the 3rd Century, even as the Emperors debased the currency so that by 200 AD the troops pay was worth less than a 1/3 of what it had been in 1 AD.
Augustus had moved the Legions from Italy to the Borders (Thus eliminated the cost of feeding the legions. This also removed the Legions far enough from Rome that the Legions could not overthrow the Emperor on a whim. The Legions seem to stay about the same till Severus (193-211) when a long slow decay set in (Mostly from the drop in pay during the pervious 200 years). By the Time of Constantine (306-337) the Roman army of the Frontier had become nothing more than local militia who very rarely moved from their bases. Diocletian had developed a new "Mobile" Army. This was divided into two parts, one based on Constantinople (after the Reign of Constantine) and the other in Milan. These were mobile and HORSE driven (From the Time of Trajan, the Roman army had moved from a Infantry based army to a more mobile horse based Army).
These "Mobile" Armies were the key to Roman Strength From about 300 AD till the Battle of Adrianople in 378 AD. With the Roman Defeat at Adrianople the Roman army had to be slowly rebuilt. It never was as strong as it had been prior to 378, and as the 400s went on it deteriorated even further. Till by 476 the Western Empire was using nothing but Barbarian mercenaries.
Now in the east a rapid rebuilt was done so that Constantinople could retake Italy in 530s (along with Southern Spain and modern Tunisian) but in the West it stayed bad till about 700 AD (With the raise of the Franks).
Now the above is what happen to the Military, we also have to understand how the base for recruiting also went down hill. In the 300s the Romans had reduced the minimal height of their Soldiers from 5'6" to 5'2". The Mail armor their wore started to be bent instead of woven together (In fact for many years it was believed that Romans did not use Mail, until examples of Mail Armor was found. The Reason for this belief was no small pieces of Mail was found in Roman Battlefields, but pieces were found in Dark Ages Battlefields. What happen is that During the Height of Rome the Romans interlocked their mail armor together so small pieces did NOT break off when hit. In the Dark Ages and the later Empire Mail stopped being interlocked and became more bent into each other. This was cheaper to make, but broke off in battle. Example of the General Deterioration of Roman Equipment from 200-500 AD.)
For a Work on the Late Roman Empire:
http://www.pillowrock.com/ronnie/romanarmy.htmFor a list of Roman Emperors:
http://www.roman-emperors.org/Now in the Hobby stores there are some nice short and accurate books on the Roman Army at various time in History. Most of these books are written by and for War gamers (and generally out of Britain). They are the best source for information for these War Gamers are more worried about HOW the Army fought than explaining why Rome fell (through they are inter-related). My point is the war-gamers tend to be less ideological about why Rome Fell than other writers.
Also one of the Comments about the late Roman Empire is the raise of what we would call the Armored Knight. The Full Armored Knight only developed twice in history, 300-700 AD and again 1300-1500. Both developed during a period of severe internal problems. The Armored Knight is effective against untrained peasants, against trained troops it is less effective. (The Armor to protect the Knight is to heavy for the Knight to operate independent of the Horse, but if the horse is given similar armor the weight is to much for the horse. Thus against trained troops who are trained to kill the horse first, the Armored Knights are a minor problem (and not worth the expense, regular Calvary is more effective). On the other hand, untrained peasants do not know that the best way to kill the knight is kill the horse first. Such untrained peasants try to kill the rider whose armor protects them, so that the Knights have time to kill the peasants first. It is interesting that the Fully Armored Knight appear when outside wars are rare (300-400 AD and 1300-1500) but peasant revolts are common. It is further interesting such knights tend to disappear whenever a real army appears (The Arabs in the 600s and the religious wars of the 1500s).
Thus in the 400s, the Roman Armored Knight was well equiped, but his infrantry may have a shield a sword and a Pike (i.e. little or no personal armor). Some Troops did retain such equipment but it was rare and as it wore out not replaced. The Short swords of the Roman Legion (used from From the time of Scipio onward), had disappeared with the lost of body Armor in the 300s. This was replaced by a Round Shield, the Pike and the Long Sword. In the late Empire Infrantry rarely won battles, it was Calvary that was the perferred arm (see above about the Armored knights).
My Two favorite comment on the invasion of the Barbarians are the following:
1. Except for the Vandals, NO Barbarian group settled in the West without first being defeated by the "Roman" Army (and than settled on lands where peasants have been in revolt).
2. In the 430s, after the death of Attila the Hun, Rome was on the March, did it go after the Goths in Southern Gaul? After the Franks in Northern Gaul? After the Barbarians in Central Gaul? After the Vandals in Spain and later modern Tunisian? No, they went after the peasants in revolt in Modern Brittany. Later on when the Leadership of Roman Britain wanted to escape from the invading Anglo-Saxons, the Romans moved them to Brittany (thus its post-Roman name). Why where they moved into Brittany? To put down the peasants in revolt.
Rome’s main concern during the 400s was to keep the peasants down. All Rome needed to do to stop the Barbarians was to armed and train the Peasants. The problem if you arm Peasants you have to give them something to fight for and the only thing the peasants would fight for would be land. The Roman Leadership had since the Time of the Grachia (162-123 BC) REFUSED to do land reform, they preferred Barbarian rule than giving up their estates. When Justinian invaded Africa and Italy in the 530s it was do to proposed land reforms by the Ruling Goths. The Romans asked Justinian to end such land reforms and he did by his invasions. Do not worry, the Lombards invaded Italy in the 570s. They brought with them land reforms and thus made their conquest of Northern Italy both easy and quick (Some Historians believe the Lombards had been invited in by Romans who finally knew land reform was needed, but could only be done by the Barbarians, thus the speed of the Lombard Invasion of the 570s).