Thomas Simson Pratt is said to have been born in 1797, the son of Captain James Pratt and his wife, Anne Simson. Probably in 1827 he married Frances Agnes Cooper. The places of his birth and marriage are unknown. Thomas and Frances Pratt had a son, Thomas, and a daughter, Anne Maria, who in 1860 married Sir Henry Barkly, then governor of Victoria, Australia.
After attending the University of St Andrews, in Scotland, Pratt served as a volunteer with his father's 56th (West Essex) Regiment of Foot in Holland in 1814, before being commissioned as an ensign in the 26th (Cameronian) Regiment. Subsequently stationed in the Mediterranean, Ireland and India, he had become a major by 1835. Serving in the China campaign of 1840–42, he was promoted to lieutenant colonel and created CB in 1841 in recognition of his leadership of the 26th. From 1843 to 1855 he was deputy adjutant general of the forces in India's Madras presidency.
Made a major general in 1858, Pratt arrived in Melbourne, Australia, to take command of the troops in the Australasian colonies in January 1860. After the disastrous defeat suffered by British forces at Puketakauere, New Zealand, on 27 June 1860, Pratt decided to take personal charge of the Taranaki campaign. He reached New Plymouth on 3 August.
His strategic problem was simultaneously to defend New Plymouth and seek a decisive victory. Throughout the war the civilian Pakeha population was besieged in the provincial capital. While many women and children were evacuated to Nelson, Pratt's attempt soon after his arrival to enforce the departure of dependants met with passive resistance. In October there were still some 900 civilians cooped up in the town. Crowded and insanitary conditions bred illness; 121 settlers died from disease during the war. New Plymouth was thus a liability as well as a vital base. Its defence tied down perhaps half the troops in the province, and the presence of hostile forces on both sides of the town limited operations in either direction. New Plymouth was never attacked, but this possibility powerfully influenced the nature of British operations.
After strengthening New Plymouth's defences, Pratt sought to bring about a decisive battle. He initially had at his disposal some 2,000 regular troops and 600 militia; his opponents, of whom half, eventually, were from Waikato, were outnumbered by about two to one. The Maori, initially reduced in numbers by the exigencies of spring planting, were, however, not amenable to combat on British terms. Although individual pa were abandoned in the face of expeditions by strong British forces, cultivations and base areas remained protected by a defensive cordon. Not even the fortuitous rout of a newly arrived Ngati Haua contingent at Mahoetahi on 6 November significantly altered the strategic situation. Local settlers criticised Pratt's caution, and suggested that irregular volunteer forces should be permitted to take on the Maori at their own game of bush fighting.
In reality, however, the Maori strategy did not amount to guerilla warfare; although they invited battle on ground of their own choosing, this was not deep in the interior. Fighting pa were built in accessible, cleared areas near the coast, some within a mile of British redoubts. Had volunteer forces campaigned in the bush, they would likely have met with no more success than they were to have in Titokowaru's War in 1868.
Economic warfare was another important element of Maori strategy. Settler property outside the fortified areas was pillaged, some 200 farms eventually being ruined. The British, who had been first to destroy property (after the 'battle' of Waireka in March 1860), continued to do so throughout the war. Between June and November more than 30 large pa and many cultivated areas were destroyed. Victory, however, was brought no closer.
Pratt's new solution, devised in concert with his staff officers, was to resort to sapping. This approach, employed against a series of pa extending south up the Waitara River and protecting a productive hinterland, dominated British strategy from 29 December 1860 until the final ceasefire on 18 March 1861. A series of redoubts (eventually numbering eight) were used as bases for advances by sap against these pa, two of which were soon evacuated. Progress was then halted by the formidable defensive work at Te Arei. When a truce came into effect on 19 March, the head of the sap was still 90 yards short of the first main line of defence, and a further position was under construction behind Te Arei.
The first Taranaki war had in fact ended, but this was far from clear at the time. Maori bases were still well protected from seizure; plunder of settler property continued; and Pratt's successor, Lieutenant General Duncan Cameron, was at first unwilling to encourage the return of civilians to the province. Subsequent events gave the lie to British claims of victory. Scarcely any combatants submitted even formally to British authority; neither the 'stolen' property nor compensation for it was forthcoming; and no Maori alleged to have killed civilians were ever handed over. Taranaki and Ngati Ruanui even seized the Tataraimaka block in lieu of the Te Ati Awa land at Waitara which had provided the pretext for the war. A military stalemate had cost some 200 casualties on each side.
Pratt returned to Victoria in April 1861, and was awarded the KCB in July. He was prominent in the colony's affairs until his return to England in April 1863. He became colonel of the 37th (North Hampshire) Regiment, was promoted to lieutenant general in 1865, and to full general in 1873. He retired in 1877, and died in Bath, Somerset, England, on 2 February 1879.
The 'whiskered and heavy-jowled' Pratt was in indifferent health during the Taranaki campaign. He was seen as brave and conscientious, but also as over timid and with a feeble and irresolute will. Such sentiments owe something to Pratt's failure to defeat a consistently under-rated foe. Richard Taylor's judgement that he 'effected as much perhaps as could have been effected with the force under his command' is probably a fairer one.