Why don't we just invade Iran?

Tactically its a more sound move than Syria. We can bring Syria under pressure easily and the educated population in Iran would be happy to see us come. Seriously if they are supplying weapons that kill our guys, let's just go stop it. Why let this get out of control? I realize they are a sovereign nation and all that, but really if they are getting our people killed we've got a right and a really good reason to put a stop to it. Naturally it would take a much better plan than Iraq and we wouldn't have to take down the whole country. Is it really a bad idea? Russia's not really going to do anything to stop us and China is too dependent on our money.