In the middle of the night January 3-4 an American air strike in Baghdad killed Qassem Soleimani, senior General of Iran's Qud's Force, arguably the second most important man in the Iranian state. The Americans blame Soleimani for masterminding hundreds of attacks on American forces.
Soleimani was not simply a "terrorist," but instead a very rare bird. He was a strategic mastermind who also knew enough about the nuts and bolts of military and paramilitary operations to actually direct them on the ground. The common folk in Iran getting uppity? Battlefield losses in Syria? Pro-Iran forces in Iraq facing popular uprisings? Tehran's answer was always the same: send in Soleimani to take control and use his command of the nexus of guns and protests and RPGs and politics and culture to rework the field of competition. He has been the Iranian fixer since the early 1990s. The "spontaneous, popular" assault on the American Embassy in Baghdad this past week was exactly his sort of operation.
And with that assault, to be blunt, the Iranians went too far. The American force profile in the Middle East region has already dropped by more than 80%, and was well on its way to zero. Consider what has happened specifically during the presidency of Donald Trump:
The Iranians shot down a $200ish million dollar recon drone. The Iranians doubled down on their Syrian and Yemeni operations. The Iranians threw a bunch of ordinance at Saudi oil infrastructure, taking more daily oil supply offline than any attack at any time in history. The Iranians deployed forces to crack down on anti-Iranian protests in Iraq. The American response to all of it was the same: nothing.
But when it comes to embassies, the Americans get twitchy. In the aftermath of Iran’s 1979 Revolution, the Iranians seized control of the American embassy in Tehran and held its staff for over a year. In the American mind that singular event is what set the United States and Iran off on their half-century of mutual recrimination and loathing. (The Iranians’ “singular event” occurred somewhat earlier.) In throwing an attack at the Embassy, the Iranians resurrected a bogeyman from America's past, all but forcing a response.
If the goal was to force the Americans to the negotiating table or speed their pullout, this move was seeped in unfettered stupidity. The Americans have proven that they’ve had active tabs on someone who really mattered. Now the Iranians have to wonder who else the Americans have eyes on. The "smart" Iranian play at this point would be to suck up the loss of Soleimani and wait for the Americans to resume their drawdown. It is not clear ego will allow that. Iran’s near-immediate pledge of “severe revenge” would certainly argue that Tehran isn’t going to quit while they’re ahead.
Should Iran wish to engage in a “constructive” conflict, there really are not good options.
US forces are mostly out of Syria, so there's no vulnerability for the Americans there. America’s Qatar deployment is their regional HQ, making it the very definition of a hard target. The only place US forces remain in a meaningful concentration is Saudi Arabia, where the Americans have put in a few thousand extra troops to protect Saudi oil fields. But if the Iranians attack those assets, the Americans are more than capable of using their naval forces in the Persian Gulf to return the favor, something that US Senator Lindsey Graham has opined is the likely next step. Specifically, Graham fingered Iranian refineries. (No one speaks for Trump but Trump, but Graham is the closest thing humanity has to a Trump whisperer.) And while Saudi Arabia has the cash and staff necessary to rebuild quickly, Iran does not. That just leaves Iraq, where attacking what’s left of the American garrisons is a great way to invite more firepower in – firepower that will all be aimed at you - rather than escorting it out.
None of which means the US can “win” either. That would take more troops. A LOT more. Keep in mind the American attempt at just Iraq took 100,000 soldiers and ultimately ended in failure. Troops in-country are now only about 5000.