My understanding is that the Iraq invasion was done mainly to test the "spread democracy" strategy, which the Bush administration believed in, and WMDs were more or less an excuse. Since that didn't work out so well, there seems to be little chance that Iran will be attacked in a similar way.

Game theoretically, physically invading a country to stop WMDs is much too costly, and not a credible threat, especially since lots of countries have already developed WMDs without being invaded.