The economic and political risks the West would face under a regional war scenario that included Iran is more than we are prepared for with our troops already on overextended tours of duty in Afganistan and Iraq, though Israel may try to force our hand in this anyway. This strategy may backfire on them in the long run should it plunge the West into a serious economic depression. The inflamed Arab passions in the region could cause the Saudi Royal family to be toppled, a geo-political nightmare for us should it ever happen.
Why? Our "alliance" is with the upper levels of the royal family, in hopes that the King can rein in his many princes, each of whom have independent hoards of wealth that are being used to finance terrorism.
If the royal family is overthrown, this can be our cue to sweep in, seize the oilfields, and strip the Saudi princes of their assets. Terrorists would then have to depend on Iran, a country historically not friendly to Arabs, for their money. Under such circumstances, would Iranians still be willing to let Ahmadinejad lead them into suicide?