Historically tanks were very poorly used in WW1 hence why they werent overly significant. The British basically through away their technologicalwonderweapon in 1916 by not using them in force and using them in poor tank terrain.
By 1918 better tactics were in use, and the British tanks were devestating. I forget the battle, but more miles of German terrain were taken in a couple of hours with the tanks then what would have taken weeks using the standard Infantry wave attacks.
Having WW1 tanks as a standalone tech wouldnt really be worthwhile, though maybe as a precursor to what is currently the main tank in Civ4 (the one with the Sherman modle).
"Cavalry", at least is the US army, and what is used in Civ 4 were the dudes on horses that spent their time fighting "Indians" and making Last Stands, etc etc. During WW2, horses were replaced with Armoured Cars, but were still refered to as Cavalry as they were used for scouting and recce work.
In the 1960s, shortly before Viatnam, the US army began experimenting with Infantry riding into battle on helicopters, and these troops became the new "Cavalry". There is an excellent book out there called "We Were Soldiers Once and Young" by Hal Moore (Mel Gibson made the crappy movie losely based on this book), and it deals with the first major battle of this new cavalry. They were even named the 7th Cav, the same unit as Custar!
In Civ 4, guys on horses morphing into helecopter gunships is a lose abstraction of the transistion of 100 years of history that it took for real life US cavalry to become airmobile Infantry in helicopters. Its just a pity they chose to represent them as Apache gunships instead of Vietnam era Huies.
[/history lesson over]
