In 1990, Kuwait was invaded and annexed by neighboring Iraq. The seven month-long Iraqi occupation came to an end after a direct military intervention by United States-led forces. Nearly 750 Kuwaiti oil wells were set ablaze by the retreating Iraqi army resulting in a major environmental and economic catastrophe. Kuwait's infrastructure was badly damaged during the war and had to be rebuilt. Kuwait has the world's fifth largest oil reserves and is one of the richest countries in the world per capita. Kuwait's oil fields were discovered and exploited in the 1930s and after it gained independence from the United Kingdom in 1961, the nation's oil industry saw unprecedented growth. Petroleum and petroleum products now account for nearly 95% of export revenues, and 80% of government income. Kuwait is regarded as the most developed country in the Arab League.