Why did the US back a UN demanding that Iraqi troops withdraw from Kuwait?

Nations didn't take chances with oil or anything else during the cold war which might risk a confrontation with the then USSR (Soviet Union). With the USSR dissolved in 1990 (the Berlin Wall came down in 1989) the US (and the UN) were free to make any demands they wanted to. When Hussain took Kuwait in 1990, that was an oil region. With no one to stop the US/UN (read Soviet Union) the US/UN was free to make those demands: "Withdraw from Kuwait...or else!" Desert Shield commenced in 1990 (military buildup to be used if and when necessary to eject Iraq from Kuwait). Desert Storm commenced on or about 15 January 1991 to formerly eject Iraq from Kuwait; this was sucessfully accomplished within approximately 45 days. DS was successfully concluded in February 1991.