A hypothetical invasion of the Islamic Republic of Iran in 2003 would probably be just the United States of America, with some limited Iraqi and British support.
Previous users are correct in stating that a war with Iran would be a much tougher political sell than Iraq, as Iran is a much more powerful and developed country in terms of military forces and infrastructure and much larger in terms of population and area. Not to mention that in 2003, Iran, although considered a black sheep in the international community, wasn't the
big bad devil that most countries make it out to be in 2011. A war with Iran would probably be Afghanistan x10.
IIRC, the Coalition in Iraq consisted of the USA, the UK, the Federated States of Micronesia and some guy called Zeke. And his dog of course.
Basically four countries and a handful of rebel groups were in the coalition.
* United States of America
* United Kingdom of Great Britain and Northern Ireland
* Commonwealth of Australia
* Republic of Poland
No mention of a bloke named Zeke and his dog, though.
After the invasion, more countries sent military forces into Iraq, although they widthdrew fairly quickly afterwards, due to terrorist attacks (
Spain) or domestic political pressure (
United Kingdom).