In 1839, when the British army advanced through the Bolan Pass towards Afghanistan, the conduct of Mehrab Khan, the ruler of Baluchistan, was considered so treacherous and dangerous as to require " the exaction of retribution from that chieftain," and " the execution of such arrangements as would establish future security in that quarter." General Wilshire was accordingly detached from the army of the Indus with 1050 men to assault Kalat. A gate was knocked in by the field-pieces, and the town and citadel were stormed in a few minutes. Above 400 Baluches were slain, among them Mehrab Khan himself; and 2000 prisoners were taken. His son, Mir Nasir Khan II was later raised to the masnad by the tribesmen and regained possession of Kalat.
In 1842, consequent upon the British withdrawal from Afghanistan, the occupied districts were returned to the Khan of Kalat. From the conquest of Sind by the British troops under the command of General Sir Charles Napier in 1843 up to 1854 no diplomatic intercourse occurred worthy of note between the British and Baluch states. In the latter year, however, under the governor-generalship of the marquess of Dalhousie, General John Jacob, C.B., at the time political superintendent and commandant on the Sind frontier, was deputed to arrange and conclude a treaty between the Kalat state, then under the chieftainship of Nasir Khan. The British negotiated with the Kalat State in 1854 and according to the terms of the treaty, British political agents were deputed to Kalat during the next twenty years. British expeditions passed through the Bolan on their way to Kandahar and Afghanistan, but up to 1876 the country was considered independent.
In 1875 Sir Robert Sandeman, the founder of the Baluchistan Province, first entered the country. The relations of Kalat with the British Government were governed by two treaties, those of 1854 and 1876. The treaty of 1876 reaffirmed the treaty of 1854. The treaty of 1854 was a treaty of alliance offensive and defensive. The treaty of 1854 was renewed in 1876 by Lord Lytton (under Sandeman's advice). The treaty of 1876 renewed these terms, but utterly changed the policy of non-intervention which was maintained by the former, by the recognition of the sirdars as well as the khan, and by the appointment of the British government as referee in cases of dispute between them. British troops were to be located in the khan's country; Quetta was founded; telegraphs and railways were projected; roads were made; and the reign of law and order established. Under the terms of the earlier treaty a subsidy of Rs. 50,000 was payable to the Khan, which was raised to 1 lakh in 1876. At the same time the Khan agreed to act in subordinate co-operation with the British Government; a British Agency was re-established at the Khan's court with certain powers of arbitration; and the presence of British troops in Kalat was permitted. The construction of railways and telegraphs and freedom of trade were also provided for. There are further agreements with Kalat in connexion with the construction of the Indo-European Telegraph, the cession of jurisdiction on the railways and in the Bolan Pass, and the permanent lease of Quetta, Nushki, and Nas1rabad.
In 1877 the cantonment of Quetta, which became the head-quarters of the Administration, was occupied by British troops, and in 1879 the administration of the district was taken over on behalf of the Khan of Kalat. The nebulous claims of Afghanistan to Sibi and Pishin were disposed of by the treaty of Gandamak in the spring of 1879. After the Afghan war, 1878-81, the districts of Pisbin, Shorariid, Duki, Sibi, and Shahrig were assigned to the British and in November, 1887, were formally constituted as British Baluchistan.
The final consolidation of the existing form of Kalat administration was effected by Sandeman's expedition to Kharan in 1883, and the reconciliation of Azad Khan, the great Naushirwani chief, with the khan of Kalat.In 1883, the districts of Quetta and Bolan were made over by the Khan to the British on aquit-rent of 25,000 rupees and 30,000 rupees respectively. In 1886, the Bori valley, in which is now the cantonment of Loralai, was occupied. In 1887, the Khetran country, now known as the Barkhan tahsil, was brought under British control; in 1889 British authority was established in the Zhob valley and Kakar Khurasan; in 1896 Chagai and Western Sinjrani were included in administered territory ; in 1899, the Nuskhi Niabat was made over by the Khan of KalAt on an annual quit-rent of 9,000 rupees ; and in 1903 the Nasirabad tahsil was ccquired from the Khan on an annual quit-rent of 117,500 rupees.