So, let's say the United States adopts a isolationist attitude in regards to the Middle East, as a result of Vietnam War syndrome, and doesn't form a coalition to liberate Kuwait from Iraq. Without American leadership, no Western country seriously considers kicking Iraq out of Kuwait with military force.
A coalition of Middle Eastern nations begins to take shape. (Syria, Saudi Arabia, United Arab Emirates, Qatar, Jordan, Lebanon). Can this coalition defeat Iraq by itself without outside military intervention?
Before anyone tells me, I know this idea is ASB, but I was just curious.
Nope.
It would be messy, but in 1990 the Saudi military is, well... nonexistent as an actual fighting force. The Iraqis can more or less drive straight to Riyadh. Ditto for the other Gulf States. The Gulf armies spent lots of money in Western high-tech equipment but are not yet trained to use it. The UAE are considered a very respectable military power now, but it was not remotely the case then.
Jordan and Syria have functioning armies, and Syria could at least challenge the still exhausted and logistically strained Iraqis, but I think Saddam still overpowers them. Jordan treaded very cautiously IOTL; they weren't particularly ready to oppose Saddam, and would even less reason to here.
Lebanon is just emerged from an absolutely devastating civil war. It can do very little.
Of course, both Iran and Israel would literally
freak out, and quickly. Israel cannot ally with Arab countries openly, but Iraqi victory is a nightmare scenario for them. Likewise, Iran may not be very willing to resume open war with Iraq so soon after the armistice, since they'd had horrific losses in the previous war... but they may easily feel compelled to do so, particularly to support Syria, which, while not yet the firm ally it would become later, is already a partner.
Assuming non-Arab countries di not intervene directly, the game changer would be what Egypt (freshly re-admitted into the Arab League) does. Egypt has the only other military force of real note in the region that could potentially stand up to the Iraqis.