Historically, both domestic and foreign powers have treated the areas known today as Iraq and Syria as a single territory. However, the differences between the two lands have been significant enough to prevent any one force from simultaneously dominating both territories for very long. This is a recurring theme going back centuries, even before the rise of Islam, and was as true for the Sassanid and Byzantine empires as it was for the Abbasid and Umayyad caliphates, and later for the rival Baath parties in Iraq and Syria in the post-colonial period. The U.S. ouster of Saddam Hussein's regime brought to power a Shia-dominated government in Iraq, which, combined with Iran's decades-old alliance with Syria, gave Tehran two friendly governments along its western border. However, this newfound position of strength was quickly jeopardized by the outbreak of the Arab Spring.