After the October 23, 1983 bombing of American and French barracks in Beirut by the Shia group the Islamic Jihad Organization that killed 241 American and 58 French personnel, parts of the American and French governments (particularly secretary of state George P. Schultz) wanted to launch retaliatory attacks against Iranian and Iranian-aligned Shia positions in Lebanon. However, secretary of defense Caspar Weinberger was against retaliation because it wasn't certain that the Iranians and/or Hezbollah were behind the attack, so there was no major retaliation, and the Multinational Force withdrew from Lebanon in February 1984. What if Weinberger had been overruled and retaliation was ordered?
The scenario that unfolds is what follows from that decision.
The Reagan Administration attributes responsibility to Iran and resolves to retaliate at a time and place of its choosing, not necessarily in Lebanon, although there are some retaliatory bombardments as there were in OTL. The withdrawal of the Marines still proceeds in the ATL, indeed it moves faster, completing by January 1st, 1984.
However, from late October the Administration demands a plan to impose a cost on the Iranians without getting into a Vietnam-like quagmire, without using ground troops and without requiring congressional approval. Beyond that, the rest is a matter of detail.
The handiest instrument of revenge against Iran at this time is Iraq, already at war with Iran.
The Administration opens talks with the Iraqis about combined operations against Iran, and US officials survey Iraq's airfield infrastructure.
By late December USAF ground crews and aircraft have been secretly deployed to Iraq.
The US and Iraqis have agreed to a limited time joint campaign against Iran. The U.S. gets use of Iraqi airfields to launch ground-based airstrikes and launch search and rescue missions. It's objective is to do visible damage to the Iranian military, its oil infrastructure and ports.
The catch is that the US campaign plan is for fewer than 80 days of strikes, and for a removal of US forces of Iraq within 85 days, to avoid tripping the requirements of the war powers act for congressional authorization to go beyond 90 days.
Although it is only a limited commitment, Saddam Hussein sees nothing not to like in the American plan, and is amenable. The U.S. explicitly tells the Iraqis it looks forward to border changes in Iraq's favor, up to and including the potential Iraqi annexation of the Arabic speaking speaking of Khuzestan.
Saddam finds diplomatic support for his territorial aims as well as the loan of US firepower attractive. Even though his infantry will have to be doing the ground advances, he is also helped with some US arms, logistics support and support by US planning staff officers to pursue the ground operation on the southern front against Iran.
The US air campaign begins operations from carriers and Iraq and other Arab Persian Gulf states on 3 January 1984.
The President explains the purpose is to strike back at the Khomeini regime for its terrorism since the hostage crisis and most recently the Iranian-supported bombings in Beirut, and it is also to curb Iranian expansionist ambitions in an area of US vital interest.
Soon after the air campaign begins, the Iraqi offensive or counteroffensive begins (I'm not sure if the Iraqis were on the offensive of defensive at the beginning of 84, or if they stood on pre-war Iraqi or Iranian territory).
The Iraqi objective is to destroy Iranian armed forces in front of them and the occupation of Khuzestan, to create a longer Iraqi coastline (and incidentally, to secure the Shatt-Al-Arab for Iraq), unite with "oppressed Arabs" and gain some of Iran's most oil rich areas.
How do things go over the next three months?
How do things go for the rest of the 20th century after that.