I have been reading up a bit on the Roman Legions and have a couple questions.
First of all why was the gladuis replaced by the spatha? The spatha was a fair bit longer than the gladius and therefore significantly harder to wield in close formation. Following the myriad successes achieved with the gladius why change?
Well the gladius was an amazing sword at the hands of a well trained legionary, but you had to had courage to use it, because in an open conflict the gladius short reach is an handicap. The gladius was perfect for tight formations, but it took a lot of time to get used to it and it need to be used by proper soldiers that had to work in unison to hack their enemies down to size. With the crisis of the third century, the training became more dull, the professionalism deteriorated, the longer sword was developed because, while less effective than the galdius if the Romans were retaining their proper formation and discipline, it was easier to train and could be used without a stellar formation and without the discipline of previous armies. It's also a lot less scary to face an enemy with a long sword than a short sword.
The spatha was more versatile in its employment- instead of predominately being a hacking and stabbing weapon, the soldiers could slash at their enemies from a longer standoff range. They did not need to close with the enemy quite as much. Plus while the gladius was useless when the formations broke, the spatha was perfect for open fighting. Ex: During the 8th and 9th centuries it was common for warriors to use two swords, a short one for when they fought in the shieldwall and another one, longer, for open fighting.
Secondly, does anyone know the reason for switching to, and then away from, the lorica segmentata? Was it more effective? Easier/harder to make and maintain?
The lorica segmentata was an amazing peace of armour. The segmentata will stop the impact of a blow better than mail, but it was very expensive to make and to maintain. The legionaries had to pay their armour, actually their first salaries were used to pay for their equipment, and segmentata was expensive so most would prefer to use the cheaper hamata or the even cheaper squamata, scale armour. The maintains was also different, to repair the hamata would take any armorer about 2 minutes with a ring, a rivet, and a couple basic tools. To repair the segmentata one would probably had to do removing and replacing several rivets, as well as replacing leather straps or making new fittings.
Plus i think is safe to compare the segmentata to the parade uniform, the armies have these days.
Thirdly, why was the rectangular scutum replaced with the more oval shields of the later legions?
The scutum lacked proper maneuverability. It was an heavy shield, more suited to pitched battles, and is use in the battlefield would be hard. But it was perfect to the type of tactics the roman used during the principate. With the substitution of the gladius for the spatha, the tactics changed. The traditional fighting style had the legionary holding the shield close to the body, using the short gladius to stab out beyond the rim at close quarters. The change to the oval, and the longer spatha and spear which happened at around the same time, suggests a more open style of fighting, with slashing blows and the enemy kept at a greater distance. The oval shield would allow a freer use of the long sword or spear.
Finally how many of these changes were made due to the changing role of the legions from a offensive juggernaut to a defensive immovable object?
The usual theory is that the roman army adopted a Defence-in-depth strategy instead of the usual roman strategy of forward defense. But I personalty think that is false. The roman didn't had a army general staff to implement such a strategy in all it's borders, more the roman ideology never changed, they keep on being aggressive about their military posture.
Virtually all forts identified as built or occupied in the 4th century on the Danube lay on, very near or even beyond the river, strikingly similar to the 2nd-century distribution.
Another supposed element of "defence-in-depth" were the comitatus praesentales (imperial escort-armies) stationed in the interior of the empire. A traditional view is that the escort-armies' role was precisely as a strategic reserve of last resort that could intercept really large barbarian invasions that succeeded in penetrating deep into the empire. But when they were implemented, 312 AD, there hadn't been an invasion for over 40th years, so i can safely assume that they were created more to provide emperors with insurance against usurpers, because they were stationed weeks, if not months, from the areas that would be attacked.
Furthermore, the late empire maintained a central feature of the forward defense of the Principate: a system of treaties of mutual assistance with tribes living on the imperial frontiers. The Romans would promise to defend the ally from attack by its neighbors. In return, the ally would promise to refrain from raiding imperial territory, and prevent neighboring tribes from doing the same.
Late Roman emperors continued major and frequent offensive operations beyond the imperial borders throughout the 4th century. Ex: Valentinian I's campaign against the Quadi in 375. Julian in 356–60 and Valentinian I in 368–74 carried out several operations across the Rhine and Danube designed to force the submission of local tribes and their acceptance of tributarii status.
I hope i could be of help, if you need to see more may i suggest my own sources.
The Roman Army : The Greatest War Machine of The Ancient World, editor Chris McNab, and the romanarmytalk and historum websites.