An attack on Iranian nuclear facilities would mostly involve air assets, primarily Air Force and Navy, that are not strained by operations in Iraq and Afghanistan. Moreover, the presence of U.S. forces in countries that border Iran offers distinct advantages. Special Forces and intelligence personnel already in the region can easily move to protect key assets or perform clandestine operations. It would be prudent to emplace additional missile-defense capabilities in the region, upgrade both regional facilities and allied militaries, and expand strategic partnerships with countries such as Azerbaijan and Georgia to pressure Iran from all directions.
Conflict may reveal previously undetected Iranian facilities as Iranian forces move to protect them. Moreover, nuclear sites buried underground may survive sustained bombing, but their entrances and exits will not.
Of course, there are huge risks to military action: U.S. and allied casualties; rallying Iranians around an unstable and oppressive regime; Iranian reprisals be they direct or by proxy against us and our allies; and Iranian-instigated unrest in the Persian Gulf states, first and foremost in Iraq.
Furthermore, while a successful bombing campaign would set back Iranian nuclear development, Iran would undoubtedly retain its nuclear knowhow. An attack would also necessitate years of continued vigilance, both to retain the ability to strike previously undiscovered sites and to ensure that Iran does not revive its nuclear program.