Saudi Arabia-United States relations refers to the bilateral relations between the Kingdom of Saudi Arabia and the United States of America, which began in 1933 when full diplomatic relations were established and became formalized in the 1951 Mutual Defense Assistance Agreement. Despite the differences between the two countries—an ultraconservative Islamic absolute monarchy, and a secular constitutional republic—the two countries have been allies. Former Presidents George W. Bush and Barack Obama have close and strong relations with senior members of the Saudi Royal Family.