Inner Cambodia

Inner Cambodia was a historical region in present-day Cambodia that was under the direct rule of Siam between 1794 and 1907. It covered much of north-western Cambodia, and included, most significantly, the cities of Phra Tabong, Siemmarat and Si Sophon. The region was ruled by a Bangkok-appointed governor who held the title Aphaiphubet, and was incorporated under the monthon administrative system in 1891, becoming known as Monthon Burapha in 1900. The territory remained under Thai control until 1907, when it was ceded to France and incorporated into French Indochina. The region was again briefly annexed by Thailand during the Second World War from 1941 to 1946.

Establishment of Siamese rule

In the late 18th century, post-Angkor Cambodia was much weakened against its neighbours Siam to the west and Vietnam to the east, who fought for influence over the smaller country. Following factional struggles in the 1770s, a pro-Siam nobleman named Baen brought Prince Ang Eng, the only male survivor of the Cambodian royal family, to take refuge in Bangkok under the Siamese King Rama I. Baen received the title Chaophraya Aphaiphubet from the Thai court, and was established as the de facto ruler in the Cambodian royal capital of Oudong.
When the Prince was allowed to return to Cambodia to assume the throne in 1794, Rama I had the northwestern area of the country, which constituted most of its border with Siam, placed under the control of Baen, who was instituted as governor of the region, based in Battambang. Siem Reap, the site of the ancient Khmer capital of Angkor, was the other major town in the region, the most fertile in Cambodia. This served a twofold purpose, strengthening Siam's control over the frontier while removing Baen from the conflict in Oudong, where his rule was deeply unpopular. The region, whose governor ruled autonomously according to Cambodian customs but reported directly to Bangkok, became known to the Thais as Inner Cambodia, while the remainder, Outer Cambodia, continued to be ruled by the Cambodian monarch, who was at various times under tributary status to Siam, Vietnam, or both.
Following Baen's death in 1809 shortly after that of Rama I, the new Thai king Rama II named Baen's son as the new governor at Phra Tabong, inheriting the Aphaiphubet title. While Khmer sources would later dispute the act, stating that the arrangement was originally understood to be limited to Baen's lifetime, it established the hereditary succession which placed rule over the region in the hands of the Abhayavongsa family for much of the following century, except for the period between 1834 and 1839, when the Thai court granted the role to the Cambodian prince Ang Im.
Phra Tabong served as an outpost from which Siam launched military expeditions into Cambodia, especially during its wars with Vietnam in 1833–1834 and 1841–1845, during the reign of Rama III. The Thai military commander Chaophraya Bodindecha, who led the campaigns, oversaw the reconstruction of the city and its fortifications from 1837 to 1838, and did so in Siemmarat as well. He also founded the towns of Mongkhon Buri and Si Sophon, the latter of which was mainly populated by Lao forced settlers following Siam's conquest of Vientiane in the 1826–1828 war.

Colonial pressures

With France's colonial expansion into Southeast Asia in the second half of the 19th century, the French protectorate of Cambodia was established in 1863. After a few years of diplomatic manoeuvring over Cambodia's tributary status, the Franco-Siamese treaty of 1867 was concluded in Paris, in which Siam recognized the protectorate and relinquished its claims to suzerainty over Cambodia, while France recognized Siam's territorial claims over Battambang and Siem Reap, including the monument of Angkor Wat. Nevertheless, the French expressed continued interest in the ruins of Angkor, which had been popularized in the Western imagination by the writings of Henri Mouhot following his travels there in 1860.
Siam's King Mongkut also showed interest in the monuments, and also in 1860 he had ordered the dismantling of a small Khmer temple for reconstruction in Bangkok. Some 2,000 labourers were conscripted for the project, which was terminated after a band of local Khmers emerged from the jungle to attack the party and murdered the Siam-employed officials overseeing the work. Later, in 1867, Mongkut would commission a different representation of Angkor in Bangkok, and had a scale model of Angkor Wat built in the royal temple of Wat Phra Kaew in the Grand Palace instead.
Despite its location in Siam, the French were able to secure permission for exploration and study of Angkor's sites, and developed Angkorian art and architecture into a symbol of Cambodian national identity. These seemingly academic undertakings also served political purposes, helping to strengthen France's colonial agenda and further the argument for the region to be returned to Cambodia.
As colonial pressure increased towards the end of the century, King Chulalongkorn implemented centralizing reforms and introduced the Monthon Thesaphiban administration system to bring Siam's fringe towns and cities and tributaries under Bangkok's direct control, effectively annexing them into Thai territory in line with the Western concept of territorial sovereignty. Inner Cambodia, now comprising the mueang of Phra Tabong, Siemmarat, Si Sophon and Phanom Sok, was established as Monthon Khamen in 1891. Phraya Maha-ammattayathibbodi was posted as commissioner to the new monthon, based in Si Sophon.
However, the conflict with France soon erupted into the Franco-Siamese crisis of 1893, which resulted in Siam being forced to cede extensive territory to France. Inner Cambodia remained under Siam's control, though the Franco-Siamese treaty of 1893 demanded that Siam demilitarize the area. With the Thai government's wariness over further French designs, little effort was made to support development and fully implement reforms in the region, which was renamed to Monthon Tawan-ok in 1899 and Monthon Burapha in 1900.
The situation continued until the Franco-Siamese treaty of 1907 was concluded, with Siam ceding control of the remaining area of Inner Cambodia to France in exchange of the towns of Trat and Dan Sai, which had been under French occupation since 1904, as well as the ending of French extraterritoriality over Asian subjects. The last governor of Battambang under Siamese rule,, relocated to Prachin Buri, where he commissioned the construction of the Chao Phraya Aphaiphubet Building, regarded as a sister building of the Governor's Residence in Battambang, which he had also commissioned just prior to the handover. Both buildings now serve as museums.

World War II

In the prelude to the Pacific theater of World War II, the nationalist government of Thai Prime Minister Plaek Phibunsongkhram invaded French Indochina in 1940 to pursue its irridentist pan-Thai ideology and reclaim what it regarded as Thailand's lost territories. The war concluded in 1941 with an armistice negotiated by Japan, in which France ceded the areas around Battambang and Siem Reap, which were incorporated as the Thai territories of Phra Tabong province and Phibunsongkhram province, respectively, as well as those now part of Preah Vihear province and Laos' Champasak province and Sainyabuli province.
Following Japan's defeat, Thailand relinquished these claims and returned the territories in 1946 as a condition for its admission into the United Nations.