In the first Gulf War it was out of necessity. Iraqi troops invaded Kuwait and Saudi Arabia. https://en.wikipedia.org/wiki/Battle_of_Khafji

Even today, other than wealth, Saudi Arabia is a much smaller country than Iraq with half its population -Saudi Arabia's population figures are inflated since over 40% are foreigners-. In 1991 Iraq was much stronger, and much more self sufficient, it could have taken Saudi Arabia in days if it weren't for the coalition forces.