During the Napoleonic Wars and associated conflicts in the Caribbean, Mediterranean, and North Sea, he served as a commander of ships' detachments, and gained his reputation for ferocity and courage.

During the War of 1812, Nicolls was posted to Spanish Florida as part of the British attempt to recruit local allies in the southern front against the United States.

He set up a base at Prospect Bluff, on the Apalachicola River, and had a sturdy fort built there, where he recruited a black and Native American Corps of Colonial Marines.

Nicolls's marines and their Creek and Seminole allies fought at Fort Bowyer and were present at the Battle of New Orleans, but the war ended in early 1815 without any attacks on their base.

He returned to Britain with a Treaty of Nicolls' Outpost he had negotiated, but failed to receive support from his government for any further aid to his erstwhile native allies.

From 1823 to 1828, he was the Commandant of Ascension Island in the South Atlantic, which was followed by a posting from 1829 to 1835, as Superintendent of Fernando Po, off the coast of Africa, an important base in the British operations against the slave trade.

Sailing from Bermuda in the summer of 1814, the expedition Nicolls commanded stopped in Spanish Havana, where it was told not to land in Florida without prior request by the Captain General, Juan Ruiz de Apodaca.

[21][22] Their fighting force having been defeated at the Battle of Horseshoe Bend in March, the few hundred Redstick Creeks still alive arrived en masse at British Post, on the verge of starvation and with no possessions other than the clothes they were wearing.

There, Nicolls regrouped at Prospect Bluff, and rallied Indians and refugee ex-slaves living free in Florida, recruiting the latter into his detached unit of the Corps of Colonial Marines.

[25] At the Battle of New Orleans on 8 January 1815, Nicolls was attached, with some of his men, to the brigade commanded by Colonel William Thornton of the 85th Regiment of Foot (Bucks Volunteers).

[28] The start of 1815 was to have seen a British offensive in the south, with the Royal Marine Battalions to advance westward into Georgia, to be joined by Nicolls and his forces from the Gulf Coast.

[30] On 15 March 1815, a U.S. Army aide-de-camp named Walter Bourke communicated to Major General Thomas Pinckney that conditions were difficult on the Georgia frontier despite efforts to reinforce American defences, and to negotiate the return of slaves who had joined the Corps of Colonial Marines under the command of Rear Admiral George Cockburn still at Cumberland Island.

Cockburn was not inclined to voluntarily hand over British military personnel who risked being returned to slavery by the Americans, and professed difficulty in communicating news of the Treaty of Ghent to Nicolls.

On his own initiative, he negotiated and presented a Treaty of Nicolls' Outpost between the United Kingdom and the Creeks and Seminoles, in which a formal alliance would have been created whereby the British would provide diplomatic support for the Indian nations.

[37] Nicolls deliberately did not remove any weapons or ordnance from the well-supplied fort, leaving it in the hands of those members of his now-disbanded Corps of Colonial Marines who chose to remain.

As Nicolls hoped, the existence of a well-armed Negro Fort (as the U.S. Army soon called it) so close to the U.S. border was an existential threat to American slavery.

[38][page needed] The Royal Marine detachment embarked on HMS Cydnus on 22 April, and were duly returned to Ireland Island in Bermuda, arriving on 13 June 1815, to rejoin the 3rd Battalion as a supernumerary company.

[note 3] In leaving West Florida, according to the U.S. Indian Agent Hawkins, Nicolls had left local forces with the arms and means to resist advancing American encroachments which were leading up to Andrew Jackson's First Seminole War.

Andrew Jackson flew a false British flag on his ship to lure them aboard, and summarily executed them in Spanish territorial waters.

While he was in America, he had the local rank of lieutenant colonel (by authority of Vice Admiral Cochrane) as he was commander of a battalion of the Corps of Colonial Marines.

[12] In the summer of 1817 Captain George Woodbine, one of Nicolls's former subordinate officers, was present in Spanish East Florida together with the former British soldier and Scottish mercenary lieutenant of Simón Bolívar, Gregor MacGregor.

[49][50] Between July and October 1818, the Niles' Weekly Register of Baltimore published portions of correspondence between Nicolls and the former auxiliary Second Lieutenant Robert Chrystie Armbrister (1797–1818) of the first "battalion" of the Corps of Colonial Marines.

[citation needed] In the correspondence, assistance was asked of Nicolls to intervene with the British government on behalf of former allies seeking asylum in Spanish West Florida from perceived American wrongdoing and injustice.

Nicolls was also busied by many infrastructure projects on the island, building roads, water tanks, a storehouse, and developing the gardens on Green Mountain.

Nineteen of the 34 men in Nicolls's first contingent died soon after their arrival, and only five of the original 47 Royal Marines who accompanied him to Fernando Po in 1829 survived two years of duty on the station.

[57] Despite his differences with Owen, Nicolls was just as determined to disrupt the slave trade, and equally energetic in his attempts to convince the British government to adopt a more aggressive stance.

Frustrated in territorial annexation schemes, he invited the West African rulers of Bimbia, Old Calabar, Camaroon, Malimba, and the Bonny to Fernando Po to form an anti-slavery alliance.

Unfinished work and efforts to provide for the welfare of liberated and displaced slave populations delayed the end of Nicolls's mandate for several months, and he did not return to England until April 1835.

This charge he denied, asserting that he had never actively encouraged slaves from nearby islands to make the dangerous crossing to Fernando Po, but that if they chose to do so, it was his duty under British law and "as a Christian man" not to return them to slavery.

[note 4] A similar assessment was said to have been made in 1815 by Lord Bathurst, the Secretary of State for War and the Colonies, who called him "a man of activity and spirit, but a very wild fellow.