Human habitation in Afghanistan dates back to the Middle Paleolithic Era, and the country's strategic location along the Silk Road connected it to the cultures of the Middle East and other parts of Asia. The land has historically been home to various peoples and has witnessed numerous military campaigns, including those by Alexander the Great, Mauryas, Muslim Arabs, Mongols, British, Soviets, and in 2001 by the United States with NATO-allied countries. It has been called "unconquerable" and nicknamed the "graveyard of empires", though it has been occupied during several different periods of its history. The land also served as the source from which the Greco-Bactrians, Kushans, Hephthalites, Samanids, Saffarids, Ghaznavids, Ghorids, Khaljis, Mughals, Hotaks, Durranis, and others have risen to form major empires.