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Today's Saudi Arabia began as a state ruled by Ibn Saud. He was a conqueror who took the key cities of Saudi and made it into one kingdom. His people started out as farmers and lived by agriculture, but in the 1930s oil was discovered in the country. This started the country's venture into petroleum production.
King Saud did not stop only at this. He started forming alliances with neighboring countries and began trading. This process is continued by the kings who inherited Ibn Saud's position. It brought funds to the country until it has transformed into the Saudi Arabia known today, one of the richest countries in the world.