The Sikh empire had its capital in Lahore, spread over almost comprising what is now northwestern Indian subcontinent. The Sikh Empire entered into a treaty with the colonial British powers, with each side recognizing Sutlej River as the line of control and agreeing not to invade the other side. Ranjit Singh's most lasting legacy was the restoration and expansion of the Harmandir Sahib, most revered Gurudwara of the Sikhs, with marble and gold, from which the popular name of the "Golden Temple" is derived. After the death of Ranjit Singh in 1839, the Sikh Empire fell into disorder. Ranjit Singh had failed to establish a lasting structure for Sikh government or stable succession, and the Sikh Empire rapidly declined after his death. Factions divided the Sikhs, and led to Anglo-Sikh wars. The British easily defeated the confused and demoralised Khalsa forces, then disbanded them into destitution. The youngest son of Ranjit Singh, named Duleep Singh, ultimately succeeded, but he was arrested and exiled after the defeat of Sikh Khalsa.