The invasion of Kuwait gave Iraq , with its own substantial oil fields , control of 20 percent of the Persian Gulf oil . Taken together , Iraq and Kuwait sat on top of some 20 percent of the world 's known oil reserves ; as an article of comparison , Saudi Arabia holds 25 percent . Saddam complained to the U.S. State Department that Kuwait had slant drilled oil out of wells that Iraq considered to be within its disputed border with Kuwait .