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. 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. The British Indian Army played a vital role in both World Wars. Years of nonviolent resistance to British rule eventually resulted in Indian independence, which was granted in 1947. Large-scale communal violence took place before and after the subcontinent partition into two separate states - India and Pakistan. The neighboring nations have fought three wars since independence. Despite pressing problems such as significant overpopulation, environmental degradation, extensive poverty, and widespread corruption, economic growth following the launch of economic reforms in 1991 and a massive youthful population are driving India's emergence as a regional and global power.