In hindsight, going to war in Iraq was a grave mistake. Iraq had no weapons of mass destruction. There's nothing new or groundbreaking about that statement.
Now the world is abuzz with talk of Iran developing nuclear weapons. A lot of people feel like this is deja vu all over again, and in a way they are right. President Bush and most of the presidential candidates seem to be on a war footing with Iran, but people don't believe the WMD argument anymore. Bush used it up in 2003 and it turned out to be false. Why believe in the Iranian nuclear threat this time around?
I believe Iran is developing nuclear weapons. I admit I believed the same thing about Iraq four years ago, but this time around I'm more convinced. For one thing, Ahmedinijad has made several statements about Iran's nuclear program, though he claims it is for peaceful purposes. Why does this oil rich country need nuclear power? Have the ayatollahs suddenly become environmentalists? Second of all, a lot more countries are warning of Iranian nukes than there were countries that supported an invasion of Iraq. Changes of leadership are not the only cause of this major difference. Both France and Germany have said they are truly concerned.
President Vladimir Putin is trying to play against the United States and Europe in this matter. He's shortsighted here. Though he acknowledged Iranian nukes would be a threat to Russia, he doesn't think they are being manufactured. He thinks it is in Russia's best interest to side with Iran on this issue, but that is extremely dangerous for his country, exactly because Iranian nukes are also threats to Russian national security.
Having said all this, I still oppose a war against Iran at this point. A war is the second worst option on the table, only being better than letting Iran get nukes. A war would be disastrous for everybody. Even with a large international coalition, such a war would be devastating to many countries and wouldn't assure the destruction of Iran's nuclear program. The United States military is stretched thin and Israel would be bombarded by missiles from the east and north, courtesy of Iran and their Lebanese subsidiary, Hezbollah.
The best option is harsh sanctions until Iran allows sufficient inspections. This is the only option that would avoid disaster, but countries like Russia and Venezuela are making the success of such a move doubtful.