Even though the U.S. wants to destroy al Qaeda and similar groups to protect itself, it also wants to protect the Arabian Peninsula and its oil – oil that the U.S. does not want to see in the hands of only one regional power. Historically, the U.S. has ensured that the Saudi royal family has been the partner of choice. There are two other countries that have been large and powerful enough to dominate the Arabian Peninsula: Iran and Iraq.
Rather than occupy Arabia to protect the flow of oil, the United States has followed the classic strategy of empire, encouraging the rivalry between Iran and Iraq, playing off one against the other to balance and thus effectively neutralize the power of each. This strategy preceded the fall of the shah of Iran in 1979, when the United States encouraged a conflict between Iran and Iraq, then negotiated a settlement between them that maintained the tension.
After the all of the shah, Iraq attacked Iran. The U.S. spent the 1980’s shifting its weight between both sides, trying to prolong the war.
Saddam tried to claim the Arab region, starting with Kuwait. America applied overwhelming force at this point. They made sure to maintain the regional balance of power, thus protecting the flow of oil from the Arab Peninsula. This was the status quo until 9/11. Bin Laden was able to entice the Americans to abandon their strategy and take the bait.
The attacks of September 11 were only marginally about the United States, and the exact nature of the American response to Bin Laden’s gambit mattered little, because any response could be used to his advantage. If the Americans did nothing, this would confirm their weakness. If the Americans responded aggressively, this would confirm that they were indeed the enemies of Islam.
Iran has mountainous borders around their 70 million people. Their topography is a fortress. Given this terrain, the Americans have tried to stir a revolution from within on multiple occasions, similar to the ones that toppled the Soviet Union.
Over the years, these attempts have always failed. But after the failures in Iraq, and to the extent that the United States could neither revive the balance of power nor leave Iran the dominant power in the Persian Gulf region, it would be natural enough for the Americans to consider some kind of attack to oust the Iranian government.