Quote Originally Posted by Cras
ehmmm how can you attack right away? you basically ask money to built an army...

perhaps you should first built it, then ask the money as your army is at the gates and after receiving it.. attack?
The RTW diplomacy isn't the most detailed or logical. From what little I know, the trigger for the diplomatic option that you will "Offer to Attack [insert faction name]" is, after the deal is done, required on your part at least one directly aggressive action (be it siege or skirmish or an epic battle).

So you'll have to at least engage in a small skirmish with the Numidians right away after you signed the deal with your fellow Romans to fulfill your promise. You might delay an actual invasion for a while but the state of war (and at least one "engagement") is required to complete your end of the bargain within that turn.

Or they might just be angry with your "extorting" diplomacy. Y'know, they're your rivals and those gold are from their personal treasuries, extorted from the Plebs, of course...