Why change to the Spatha?

[Sean Manning]

I'm not as up on my later roman history as I should be, but perhaps those of you out there who are can help me out because I've always wondered this. Why did the Roman army start adopting the Spatha as their sidearm and stop using the Gladius? Why did they stop using such an efficient and feared sword for the longer version? What occured to cause this change in preference?

Perhaps it was either a Germanic fashion, or resulted in a shift from major battles to small skirmishes with a more open style of fighting. But as with many things, the Romans weren't courtious enough to leave us an explanation for why the change happened.

A lot depends on how you envision the tactics of an Imperial legion though (eg. 3' or 6' of width per man).