We don't have to invade but we should not do as we did after gulf war 1 and leave insurgents high and dry.
Sorry, but if the insurgents are fundamentalist fanatics (as they are) they most certainly should be left high and dry. As I have noted in other messages, there are much preferable avenues for regime change available in Iran. The pro-Democracy student protestors need to be supported, and encouraged in their intentions to make the strike planned for this July a massive and determined last push to topple the mullahs.
If, and only if, that fails, then we might need to think about getting ugly and maybe dealing with somewhat unsavory elements, but that should only be a last resort in preference to worse alternatives.