Iran sits at the crossroads of the Islamic world. Linking the Middle East, the Caucasus and Central and South Asia, Iran has struggled to balance the benefits and risks of its geographic position. Iran's core is located in the western Zagros mountain range. From this secure geography, the beginnings of the Persian Empire spread throughout Iran's mountainous topography securing the Alborz, the southern Zagros and much of the Iranian plateau. Iran's primary geographic challenge has been to secure itself from the many external threats on its borders. Arabs, Mongols and Turks all conquered ancient Persia at various times, prompting Persia to expand its territorial control whenever possible to establish a buffer to protect its core. At its height, the Persian Empire stretched from the Eastern Mediterranean to the Hindu Kush mountains, bridging modern-day Europe and Asia. Echoes of this former empire can be seen even today, with Iran's support for Hezbollah in Lebanon, the al Assad regime in Damascus and the Shiite-led government in Iraq. Iran has also struggled to unite and control the various language and ethnic groups located within its core territories, a process impeded by the country's difficult terrain. Modern-day Tehran is aided by its large hydrocarbon reserves; Iran boasts the fourth-largest oil reserves and largest natural gas reserves in the world. The 20th century saw British, Russian and American interests competing not only to control Iran's strategic geographic location, but its significant energy reserves as well. This modern-day reliance on energy revenues has resulted in Iran's focus on securing the Strait of Hormuz and expanding control over the Persian Gulf to secure its core territories from the threat of outside invasion.