General Sir Brent Spencer GCB (1760 – 29 December 1828) was an officer in the British Army during the American Revolutionary War and French Revolutionary Wars. During the Peninsular War he became Lord Arthur Wellesley's second-in-command on two occasions. He fought at Vimeiro and testified in Wellesley's favor at the inquiry following the Convention of Cintra. He led a division at Bussaco and two divisions at Fuentes de Onoro. After the latter action, he had an independent command in northern Portugal. Wellesley, now Lord Wellington, was not satisfied that Spencer was up to the responsibilities of second-in-command and he was replaced by Thomas Graham. Miffed, Spencer left Portugal and never returned. He became a full general in 1825.
Spencer became a commissioned officer in 1778. He fought with great credit in the West Indies in 1779–1782 during the American Revolutionary War and again in 1790–1794 during the War of the First Coalition. Promoted to brigadier general, he participated in the unsuccessful 1797 Santo Domingo campaign against Toussaint Louverture. In 1799, he led the 40th Foot during the Anglo-Russian invasion of Holland. This campaign included the battles of Bergen and Castricum.
The Dos de Mayo Uprising of Spain against Napoleon found Spencer in command at Gibraltar. On his own initiative he sailed for Portugal and arrived at Mondego Bay on 5 August 1808. The force that he brought with him were 946 men of the 6th Foot, 806 soldiers of the 29th Foot, 874 troops of the 32nd Foot, 948 men of the 50th Foot, 929 soldiers of the 82nd Foot, and 245 artillerists. All infantry units consisted of 1st Battalions, except the 29th which was not organized into battalions. At Mondego Bay he joined his force to the 8,123 troops that Lord Arthur Wellesley brought from Cork in Ireland.
At the Battle of Vimeiro in 1808, Spencer was Wellesley's second-in-command. He was made a Knight Companion of the Order of the Bath on 26 April 1809. He commanded the 1st Division at the Battle of Bussaco and in the 1810-1811 French invasion of Portugal. At the Battle of Fuentes de Onoro in 1811, Spencer, now a lieutenant-general, temporarily led both the 1st and 3rd Divisions.
Later that year, he was replaced by Thomas Graham, 1st Baron Lynedoch because of pessimistic letters which he had sent back to England. In 1825, Spencer was promoted to full general.
Wellington wrote of Spencer, "He was exceedingly puzzle headed. He would talk of the Thames for the Tagus."
After having been in succession Colonel of the 9th Garrison Battalion (1806–08) and the 2nd West India Regiment (1808–09) and Colonel-Commandant of the 2nd Battalion of the 95th Regiment of Foot (1809–16) and the 1st Battalion of the Rifle Brigade (1816–18), he was given the colonelcy of the 40th (2nd Somersetshire) Regiment of Foot from 1818 until his death in 1828.
Relationship with Princess Augusta
Spencer was romantically involved with Princess Augusta Sophia of the United Kingdom, the second daughter of George III of the United Kingdom and Charlotte of Mecklenburg-Strelitz starting in about 1803. According to Augusta the couple conducted their relationship with great discretion as they were not of equal rank; news of such an attachment was feared to disturb the King's precarious mental health. Augusta was known to have requested permission from the future George IV of the United Kingdom (her brother and then Prince Regent) to marry General Spencer, but it is unknown if this occurred. It is believed the couple remained together until Spencer's death in 1828. 
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- Hansard 1803–2005: contributions in Parliament by Brent Spencer
|Parliament of the United Kingdom|
|Member of Parliament for Sligo Borough
|New regiment||Colonel of the 103rd Regiment of Foot
Eyre Power Trench
|Colonel of the 2nd West India Regiment
Sir George Beckwith
|New regiment||Colonel-Commandant of the 2nd Battalion,
95th Regiment of Foot
Sir William Stewart
|Colonel-Commandant of the 1st Battalion,
Sir George Osborn
|Colonel of the 40th Regiment of Foot
Sir James Kempt