Why did the Roman army base on infantry?

[Michael Kerr]

I agree with you about Romans reliance on infantry but although cavalry played their part in the process in the dismembering of the Roman Empire I think overextension, civil wars and switching to a form of static defence based on the Rhine and Danube rivers in Europe really sounded the death knell of Rome. Stupidity and greed by commanders like Crassus also didn't help. He had the Gallic Cavalry led by his son and was expecting help from Armenia in cavalry. Although it wouldn't have been easy leading his army through Armenia as it was hilly country if he did he would have negated the Parthian's advantage of cavalry as well as intimidating the Armenian King into supporting him but he chose what he thought would be the easier route. Valens was jealous of Gratian and thought he was only attacking infantry at Adrianople hoping to get all the credit and not share it with Gratian. Yet in 40BC Ventidius defeated the Parthians at battle of Cyrrhestica where he learnt by Crassus mistakes and beefed up his legions with archers and slingers and chose more favourable ground (hill country). But using a sporting analogy "You play to your strengths" and infantry was their strength. But you are right about sieges as military engineering was another of their strengths. I think that is why they had so much trouble with the Sarmatians as they travelled in their wagons and so the Romans could not lay siege to their cities and towns because they moved with the horsemen. Good topic.
Regards
Michael Kerr