The Indus Valley civilization, one of the world's oldest, flourished during the 3rd and 2nd millennia B.C. and extended into northwestern India. Aryan tribes from the northwest infiltrated the Indian subcontinent about 1500 B.C.; their merger with the earlier Dravidian inhabitants created the classical Indian culture. The Maurya Empire of the 4th and 3rd centuries B.C. - which reached its zenith under ASHOKA - united much of South Asia. The Golden Age ushered in by the Gupta dynasty (4th to 6th centuries A.D.) saw a flowering of Indian science, art, and culture. Islam spread across the subcontinent over a period of 700 years. In the 10th and 11th centuries, Turks and Afghans invaded India and established the Delhi Sultanate. In the early 16th century, the Emperor BABUR established the Mughal Dynasty, which ruled India for more than three centuries. European explorers began establishing footholds in India during the 16th century. By the 19th century, Great Britain had become the dominant political power on the subcontinent and India was seen as the "Jewel in the Crown" of the British Empire. The British Indian Army played a vital role in both World Wars. Years of nonviolent resistance to British rule, led by Mohandas GANDHI and Jawaharlal NEHRU, eventually resulted in Indian independence in 1947. Large-scale communal violence took place before and after the subcontinent partition into two separate states - India and Pakistan. The neighboring countries have fought three wars since independence, the last of which was in 1971 and resulted in East Pakistan becoming the separate nation of Bangladesh. India's nuclear weapons tests in 1998 emboldened Pakistan to conduct its own tests that same year. In November 2008, terrorists originating from Pakistan conducted a series of coordinated attacks in Mumbai, India's financial capital. India's economic growth following the launch of economic reforms in 1991, a massive youthful population, and a strategic geographic location have contributed to India's emergence as a regional and global power. However, India still faces pressing problems such as environmental degradation, extensive poverty, and widespread corruption, and its restrictive business climate is dampening economic growth expectations.