I've been reading the http://fallout.wikia.com/wiki/Fallout_Bible_3, and I noticed that the timeline entry for August 2065 mentions how "Increasing need for mobility in the United States mechanized cavalry leads the military to focus the efforts on creating a man-based tank - essentially, a two-legged walking armored unit: Power Armor."

I'm something of a giant robot nut, so I was wondering, by "mechanized cavalry", do they perhaps mean "mecha"? Or is the mechanized cavalry just regular tanks (or Mister Gutsies?)

Because if they mean mech, then Fallout needs one of these: http://flyingdebris.deviantart.com/art/Mad-Max-mech-48770885

They probably mean Tanks, APCs, Armored Cars etc. Just like now, how the US has mechanized units. Which would be: Bradley's, Humvee's, and Abrams, and Cougars

Ah, I see. Well, either way, I suppose they do have Liberty Prime.

Easy mistake to make I know with a name like that, but

Noun 1. mechanized cavalry - an armored unit of a modern army equipped with motor vehicles

http://www.thefreedictionary.com/mechanized+cavalry

True, but Liberty Prime came to late in the war to be used. I think it says something about it on the Terminal infront of Prime when he's in the Citadel.

The Mechanized Cavalry, might have actually been it's own division(similar to the 1st Cavalry Division, 1st Armored Division etc.). Or on the other hand, it might just have become the de jure term to refer to armored units within in the US Army.

It would be very cool to have mech or any sort of armor clad vehicle.

Another point, perhaps the armored doctrine of the US Army changed significantly, and with it came an emphasis on a more mobile armored force, rather than that of the "old standard"(ie: the MBT). With that being said, if we are to assume, that the United States moved away from using MBT's and instead relied upon a mixture of Light Armored Vehicles(with also a high reliance on both a strong anti-armor doctrine and air assets), then perhaps it would explain the need for power armor, since when the US went to war with Communist China, it would be safe to assume, that the PRC was still using some variation of the T-55, and with the US relying on a lighter/leaner armored force, it would make sense, that power armor would have been needed to give the "Mech. Cav." the extra punch they needed to go toe to toe with the PRC's rather large force of outdated MBT's.