Keep in mind the last time the US funded a public engagement against Iran (by bankrolling Iraq and feeding Saddam with weapons), it resulted in a draw. A future engagement is entirely winnable by the US, but good lord at what cost? We're talking an engagement that might potentially run a century, at least if Persian grudges are anything to consider.
Disclosure: Persian background. Not at all in favor of the current Iranian government to the point where I wouldn't even bother flying to Iran to pay family a visit, but I've got enough exposure to the culture to know exactly why such an engagement will never end. Color me overly optimistic, but I feel like the Obama approach was working.