He was encouraged to think that the US would be neutral about Kuwait, by the ambiguous answers of the US Ambassador to Iraq. He was completely taken aback when the US objected, as it did.
As you have pointed out, what a terrible strategic error it was, for the US to make. This is the key to the collapse of their Middle East policy; under that arch-moron Trump it has been reduced to a dogged defence of Jared Kushner's patrons, the Netanyahu clique in Israeli politics. The US has destroyed its own influence in Iran, in Iraq, in Syria, and in Lebanon; it has weakened itself in with the Saudis, who were a dicey proposition at the best of times, and it worked very hard to alienate Turkey: such hard work always has some effect. Now only the fringe states of the Gulf, the Bahrains and the Qatars, the UAE and Oman are left in support, as is Jordan. If, instead, the US had nursed Iraq back to normalcy, using Saddam's sons as leverage, she would have had a strong ally to counter the Iranians, to dominate and checkmate the Saudis, to support Israel by turning a blind eye and to build bonds with Turkey and form a larger market for the resurgent Turkish military sector.