Quick and dirty : make a mistake. Any strategical mistake glaring enough (lone unit far from the main force, unsupported missiles etc...) will usually prompt the AI to try and exploit it.

Or you can just use your generals as bait : walk them to the enemy, then quickly turn back when they move, as spears will usually try and charge static cav.

If still they won't move... well, you have heavy cav versus unmoving footmen, charge their backs and be done with it :)