Following the outbreak of World War I in 1914, the British Raj used Manama as a military base of operations during the Mesopotamian campaign. Prompted by the presence of oil in the region, the British political agency in Bushire concluded an oil agreement with the Hakim to prohibit the exploration and exploitation of oil for a five-year period. In 1919, Bahrain was officially integrated into the British empire as an overseas imperial territory following the Bahrain order-in-council decree, issued in 1913. The decree gave the resident political agent greater powers and placed Bahrain under the residency of Bushire and therefore under the governance of the British Raj. The British pressured a series of administrative reforms in Bahrain during the 1920s (a move met with opposition from tribal leaders), during which the aging Hakim Isa ibn Ali Al Khalifa was forced to abdicate in favour of his reform-minded son Hamad ibn Isa Al Khalifa. A municipal government was established in Manama in 1919, the Customs office was reorganised in 1923 and placed under the supervision of an English businessman, the pearling industry was later reformed in 1924. Earnings from the customs office would be kept in the newly created state treasury. Civil courts were established for the first time in 1923, followed by the establishment of the Department of Land Registration in 1924. Charles Belgrave, from the Colonial office, was appointed in 1926 by the British to carry on further reforms and manage administration as a financial advisor to the King. He later organised the State Police and was in charge of the Finance and Land departments of the government.