Siamese claims over 'Siamese Laos'. Siam sent another mission to Paris to settle disputes. "Aubaret Treaty" was finally ratified in Paris in July 1867, in which Siam officially ceded Cambodia but retained northwestern Cambodia including Battambang and Siemreap, which would later be ceded in 1907. Western imperialism introduced Siam to new concept of border demarcation and territorial proclamations. In pre-modern Southeast Asia, borders between polities were ill-defined. Traditional Siamese government had authorities only in cities, towns and agricultural areas with mountains and forests largely left alone, unreached by authorities. In the era of colonialism, border claims and mapmaking were keys to Siam's standing against colonial encroachments. British and Siamese delegates met at Tenasserim Hills in 1866 to explore and define Anglo-Siamese borders between Siam and British Burma from Salween River to Andaman Sea, becoming modern Myanmar-Thailand border as the treaty was signed in 1868. Regency of Si Suriyawong When King Mongkut ascended the throne in 1851, he appointed his younger brother Pinklao as Vice-King or Second King of the Front Palace, giving Pinklao immense powers. Vice-King Pinklao predeceased King Mongkut in 1866. After demise of his peers, Chaophraya Si Suriyawong (Chuang Bunnag) emerged as the most powerful nobleman. King Mongkut took a trip to observe solar eclipse at Prachuap Khirikhan but contracted malaria and died in October 1868. His fifteen-year-old son Chulalongkorn was confirmed to succeed the throne under regency of Si Suriyawong. Si Suriyawong unprecedentedly made Wichaichan, son of Pinklao, Vice-King of the Front Palace and heir presumptive without Chulalongkorn's consent. His regency was the time when power of the Bunnags reached apex. The young king Chulalongkorn, who had been educated by Western teachers including Anna Leonowens and who was then powerless under sway of the Bunnag regent, spent his early reign learning and observing. Chulalongkorn visited Singapore and Dutch Java in 1871 and British India in 1872 where he learned about Western colonial administrations, becoming the first Siamese monarch to travel aboard. Chulalongkorn formed the Young Siam Society, composing of liberal Westernizing young princes and noblemen who aimed at state financial reforms and abolition of government-regulated manpower control for development of economy, people and kingdom and also to consolidate royal power through centralization. When King Chulalongkorn reached the age of twenty in 1873, the regency ended as Si Suriyawong was rewarded with highest rank of Somdet Chaophraya, becoming Somdet Chaophraya Si Suriyawong. Under ineffective Chinese tax collector system,