a vassal of Gia Long. Gia Long responded by sending an ambassador bearing the book of investiture, together with a seal of gilded silver. In 1812, Ang Chan refused a request from his brother Ang Snguon to share power, leading to a rebellion. Siam sent troops to support the rebel prince, hoping to enthrone him and wrest influence from Gia Long over Cambodia. In 1813, Gia Long responded by sending a large military contingent that forced the Siamese and Ang Snguon out of Cambodia. As a result, a Vietnamese garrison was permanently installed in the citadel at Phnom Penh, the Cambodian capital. Thereafter, Siam made no attempts to regain control of Cambodia during Gia Long's rule. Napoleon's aims to conquer Vietnam as a base to threaten British possessions in India never materialized, having been preoccupied by vast military ambitions on mainland Europe. However, France remained the only European power with permanent spokesmen in Vietnam during his reign. Trade relations Pigneau's aborted deal with France allowed Gia Long to keep his country closed to western trade. Gia Long was generally dismissive of European commercial overtures. This was part of a policy of trying to maintain friendly relations with every European power by granting favors to none. In 1804, a British delegation attempted to negotiate trading privileges with Vietnam. It was the only offer of its kind until 1822, such was the extent of European disinterest in Asia during the Napoleonic Wars. Gia Long had purchased arms from British firms in Madras and Calcutta on credit, prompting the British East India Company to send John Roberts to Huế. However, Roberts's gifts were turned away and the negotiations for a commercial deal never started. The British then made a request for the exclusive right to trade with Vietnam and the cession of the island of Cham near Faifo, which was rejected, as were further approaches from the Netherlands. Both of these failed attempts were attributed to the influence of the French mandarins. In 1817, the French Prime Minister Armand-Emmanuel du Plessis dispatched the Cybele, a frigate with 52 guns to Tourane (now Da Nang) to "show French sympathy and to assure Gia Long of the benevolence of the King of France". The captain of the vessel was turned away, ostensibly on grounds of protocol for not carrying a royal letter from the French king. Gia Long kept four French officers in his service