In the early 20th century, the Arabian Peninsula was a large desert region. The only people who lived there were some small Bedouin tribes. One of these tribes was the Saudi family. In 1901 a young man named Abdelaziz Ibn Saud re-established the country his ancestors had before. In 1915, the British joined Ibn Saud's enemy, Sharif Hussein, and war broke out in 1918. Ibn Saud was able to get more land during the war so his country grew bigger.
In 1925 the British withdrew their support for Sharif Hussein and Ibn Saud attacked gaining more land and increasing the size of his country. Ibn Saud was now King of Hejaz and in control of the holiest places in Islam, Mecca and Medina. Ibn Saud decided that Wahhabism, a sect of Islam, would be the main religion in Saudi Arabia.
In the 1930s oil was discovered in Saudi Arabia. Ibn Saud asked the Americans for help in getting the oil because the king of Iraq and Jordan, the son's of Sharif Hussein, were partners with the British. In 1939 the Americans began helping Saudi Arabia drill for oil and make money.