Why?
Saddam attacked Kuwait because he thought he'd get away with it, a tiny country, dangerously close to a 'rogue state'. Saddam thought if he annexed the lands, (and, more importantly, the oils), of Kuwait either, no-one would care, or if they did care, they would tolerate it because an Iraqi-controlled Kuwait would be more stable than an Iranian-controlled Kuwait.
It was an arrogant miscalculation.
But Saddam wasn't so arrogant that he thought people would kowtow to his whim if he were to invade the largest country in the Middle East. Sure, in a military conflict between just the two countries, Iraq would easily win, but Saddam would have to be stupid to believe that no-one would come to the Saudis' aid.