It's fairly common knowledge that the U.S. invasion of Iraq was illegal under international law. Law that the United States has signed and agreed to comply with. There's a decent summary here. What many people don't realize though, as shown at the preceding link, is that it's not just illegal to invade a country without going through the proper procedures. It's also illegal to threaten another country under normal circumstances, such as when a country is not engaging in threatening behavior towards you, etc.
Iran has in no way threatened the United States, so threats against Iran are illegal. Dick Cheney and other high ranking officials have repeatedly threatened Iran (See Article 3 of House Resolution 333, which was articles of impeachment against Dick Cheney). This is a threat that is not to be taken lightly of course as the United States has the capability and has demonstrated the propensity to in fact carry out such threats.
We must demand that laws apply equally to the powerful and the weak.