The mountains of Afghanistan are among the most hazardous terrains for military operations. This topography has long enabled Afghans to successfully resist much more powerful conventional armies, as it did when Alexander the Great tried to conquer Afghanistan in the fourth century B.C. and again when Europeans undertook conquest attempts in the 1800s. After the Taliban were removed from power, they retreated to these mountains to stage their resistance against the new Afghan government and NATO forces. Afghanistan's mountains, which run east to west through the country, are the westernmost extension of the Himalayan mountain system. The mountains consist of extremely elevated mountain ranges, such as the Safed Koh range along the Pakistani border and the Hindu Kush range to the north, which contain some of the highest peaks in the world and very rugged, steep topography.