Once you get into the successor states, cavalry declined in importance, and they tended to rely on phalanxes by themselves, pluse elephants, massively reducing the capabilities of the army, as the phalanx, while formidable, needs support. This said, it required cavalry to be truly effective - the point of the phalanx wa to act as an 'anvil2 for the cavalry's 'hammer', which is ALWAYS how Alexander used them (in fact, he used pretty much the same tactic in every battle.screen the left with lights, central phalanx, heavy charge on the right). A group of phalanxes acquired considerable greater flexibility, as the could adopt a number of formations such as wedges, refused flanks, boxes etc, and when the phalangites were well drilled, such as under alexander, the formation of the phalanx itself (the Syntagma) was flexible. By itself, it could pulverise most things in front of it, but was (obviously, and as has been said before) vulnerable to the flanks and rear.

The Phalanx, in order to be successful, really had to be use as part of a combined arms force. Alas, I have lost the second one, so can't be more specific :( I recommend reading Warfare in the Ancient world by Brian Todd Carey, and a book which I think is called 'Warfare in the classical world', with hoplite's face on the front.

Please excuse spelling ffee gives me jittery hands lol.