Afghanistan is a landlocked nation at the crossroads of Central and South Asia, officially known as the Islamic Republic of Afghanistan. Afghanistan is bordered on the east and south by Pakistan; on the west by Iran; on the north by Turkmenistan, Uzbekistan, and Tajikistan; and on the northeast by China. It is a mountainous country with plains in the north and southwest, covering 652,000 square kilometers. Kabul is the capital and the largest city in Afghanistan. Its population is estimated to be about 32 million people, with ethnic Pashtuns, Tajiks, Hazaras, and Uzbeks making up the majority. At least 50,000 years ago, humans lived in what is now Afghanistan. Settled life first appeared in the area 9,000 years ago, eventually developing into the Indus, Oxus, and Helmand civilizations of the 3rd millennium BCE. Indo-Aryans migrated to Gandhara through the Bactria-Margiana area, followed by the rise of the Iron Age Yaz I culture, which has been linked to the culture depicted in the Avesta, Zoroastrianism's ancient religious texts. In the 6th century BCE, the city, then known as "Ariana," fell to the Achaemenid Persians, who invaded the areas to their east as far as the Indus River. Invading the area in the 4th century BCE, Alexander the Great married Roxana in Bactria before embarking on a campaign in the Kabul Valley, where he encountered opposition from the Aspasioi and Assakan tribes. The Greco-Bactrian Kingdom became the Hellenistic world's easternmost point. Buddhism and Hinduism flourished in the area for centuries after the Mauryan Indians conquered it. The Kushan emperor Kanishka, who ruled from his twin capitals of Kapisi and Puruapura, was instrumental in Mahayana Buddhism's expansion to China and Central Asia. The Kidarites, Hephthalites, Alkhons, Nezaks, Zunbils, and Turk Shahis are among the Buddhist dynasties that originated in this area. The Saffarid, Samanid, Ghaznavid, and Ghurid dynasties introduced Islam to Sassanian-held Herat and Zaranj in the mid-7th century, and the Saffarid, Samanid, Ghaznavid, and Ghurid dynasties achieved complete Islamization between the 9th and 12th centuries. The Khwarazmian, Khalji, Timurid, Lodi, Sur, Mughal, and Safavid empires later dominated parts of the area. The Hotak dynasty, whose founder Mirwais Hotak proclaimed southern Afghanistan independent in 1709, is responsible for the current state of Afghanistan's political history. Ahmad Shah Durrani established the Durrani Empire in 1747, with Kandahar as its capital. The Durrani capital was relocated to Kabul in 1776, while Peshawar was designated as the winter capital; the latter was captured by Sikhs in 1823. Afghanistan served as a buffer state in the "Great Game" between British India and the Russian Empire in the late 1800s.