What was the shortest war fought in United Sstates history?

The Spanish-American war lasted from April 25, 1898 to August 12, 1898 (when the ceasefire was signed), which is 110 days. The formal treaty ending the war was signed about four months later.

https://history.state.gov/milestones/1866-1898/spanish-american-war

The length of the 1990 Gulf War depends on when you consider it to have started - on August 2 when Iraq invaded Kuwait, or on 15 January when the UN ultimatum was not met, or on 17 January when full-scale warfare began. The latter dates give it a length of 43-45 days before the end of hostilities on 28 February. The earlier date makes it much longer.

https://www.awm.gov.au/atwar/gulf/

The first Gulf was in 1991 and lasted from January 15th to March 1st. I remember those 45 days extremely well, as my grandfather died some ten days after it ended.

I find it quite ironic that the first Gulf war ended on the Jewish holiday of Purim, and the second started on the very same holiday last year.