After Vietnam under Gia Long gained control over Cambodia in the early 19th century, a Vietnamese-approved monarch was installed. Minh Mạng was forced to put down a Siamese attempt to regain control of the vassal as well as an invasion of southern Vietnam which coincided with rebellion by Lê Văn Khôi. The Siamese planned the invasion to coincide with the rebellion, putting enormous strain on the Nguyễn armies. Eventually, Minh Mạng's forces were able to repel the invasion as well as the revolt in Saigon, and he reacted to western aggression by blaming Christianity and showing hostility, leading to the European powers asserting that intervention was needed to protect their missionaries. This resulted in missed opportunities to avert future colonisation through having friendly relations, since strong opposition was raised in France against an invasion, due to the costs of such a venture. After China was attacked by Britain in the Opium War, Minh Mạng attempted to build an alliance with European powers by sending a delegation of two lower rank mandarins and two interpreters in 1840. They were received in Paris by Prime Minister Marshal Soult and the Commerce Minister, but they were shunned by King Louis-Philippe. This came after the Society of Foreign Missions and the Holy See had urged a rebuke for an "enemy of the religion". The delegation went on to London, with no success.