King Narai's reign saw a major expansion of diplomatic missions to and from Western powers, most notably France, England, and the Vatican. Missions were also sent and received from Persia, India and China, as well as other neighbouring states. Another notable feature of Narai's reign was the unprecedented influence of foreigners at the Siamese court, embodied in the meteoric rise of Constantine Phaulkon, a Greek adventurer who would eventually hold the modern equivalent of the post of Prime Minister.
King Narai especially sought to expand relations with the French, as a counterweight to Portuguese and Dutch influence in his kingdom, and at the suggestion of his Greek councillor Phaulkon. Numerous embassies were exchanged in both directions. A first Siamese ambassador to France was sent in the person of Phya Pipatkosa on board the Soleil d'Orient, but the ship was wrecked off the coast of Africa after leaving Mauritius, and he disappeared. A second embassy was sent to France in 1684 (passing through England), led by Khun Pijaiwanit and Khun Pijitmaitri, requesting the dispatch of a French embassy to Thailand. They met with Louis XIV in Versailles. In response, Louis XIV sent an embassy led by the Chevalier de Chaumont. A second Thai embassy to France was led by Kosa Pan in 1686.
These first exchanges led to a major involvement by the French with the dispatch of an embassy in March 1687 organized by Colbert. The embassy consisted of a French expeditionary force of 1,361 soldiers, missionaries, envoys and crews aboard five warships, and brought the Siamese embassy home. The military wing was led by General Desfarges, and the diplomatic mission by Simon de la Loubère and Claude Céberet du Boullay, director of the French East India Company.
Desfarges had instructions to negotiate the establishment of troops in Mergui and Bangkok rather than the southern Songkla, and to take these locations if necessary by force. King Narai agreed to the proposal, and a fortress was established in each of the two cities, which were commanded by French governors. Desfarges was in command of the fortress of Bangkok, with 200 French officers and men, as well as a Siamese contingent provided by King Narai, and Du Bruant was in command of Mergui with 90 French soldiers. Another 35 soldiers with 3 or 4 French officers were assigned to ships of the King of Siam, with a mission to fight piracy.
The diplomatic mission, however, achieved little apart from the reaffirmation of the 1685 commercial treaty. The Jesuit Father Tachard had obtained secret instructions from Seignelay, which allowed him to deal directly with Phaulkon. Hopes for the conversion of King Narai to Catholicism, which had largely motivated the embassy sent by Louis XIV, did not materialize.
The disambarkment of French troops in Bangkok and Mergui led to strong nationalist movements in Siam directed by the Mandarin and Commander of the Elephant Corps, Phra Petratcha. By 1688 anti-foreign sentiments, mainly directed at the French and Phaulkon, were reaching their zenith. The Siamese courtiers resented the dominance of the Greek Phaulkon in state affairs, along with his Japanese wife and European lifestyle, whilst the Buddhist clergy were uneasy with the increasing prominence of the French Jesuits. The courtiers eventually formed themselves into an anti-foreign faction. It is also notable, however, that other foreigners who had established themselves in Ayutthaya before the French, in particular the Protestant Dutch and English as well as the Persians, resented the growing political and economic influence of the Catholic French. Even other established Catholic factions, such as the Portuguese, had reason to resent the French presence, a violation of the Treaty of Tordesillas. The increasing French influence not only increased competition but was also an unwelcome reminder of the declining fortunes of Portugal.
Matters were brought to a head when King Narai fell gravely ill in March 1688.
In April 1688, Phaulkon requested military help from the French in order to neutralize the plot. Desfarges responded by leading 80 troops and 10 officers out of Bangkok to the Palace in Lopburi, but he stopped on the way in Ayutthaya and finally abandoned his plan and retreated to Bangkok, fearing that he could be attacked by Siamese rebels and troubled by false rumors that the king had already died. Desfarges could have eliminated the conspiracy at this point if he had pursued his mission towards Lopburi, but his judgement failed him, partly influenced by the false rumours spread by Véret, the Director of the French East India Company in Ayutthaya.
On May 10, the dying King Narai, aware of the coming succession dispute, called together his closest councillors – Phaulkon, Phra Petracha and Mom Pi – and nominated his daughter, Kromluang Yothathep, to succeed him. The three councillors were to act as regents until the princess took on a partner of her choice from one of the two Siamese councillors.
Far from calming the situation, Narai's decision spurred Petracha to act. With Narai essentially incapacitated by his illness, Petracha was given a free hand to stage a coup d'etat with the support of a resentful court as well as the Buddhist clergy. The events spurred Petracha to execute the long-planned coup immediately, initiating the 1688 Siamese revolution. On May 17-18, 1688, King Narai was arrested, and on June 5 Phaulkon was executed. Six French officers were captured in Lopburi and mobbed, one of them dying as a result. Many members of Narai's family were assassinated (the king's brothers, his successors by right, were killed on July 9), and King Narai himself died in detention on July 10-11. Phra Petratcha was crowned king on August 1. Kosa Pan, the 1686 former ambassador to France, was one of the strongest supporters of Petratcha, and became his Minister of Foreign Affairs and Trade.