In 1793 Great Britain entered the French Revolutionary Wars, and Nelson was given command of the British ship Agamemnon. He served in the Mediterranean, fighting at Toulon and helping to capture Corsica. At Calvi he lost the sight in one eye. Under John Jervis, later earl of St. Vincent, he was largely responsible, acting boldly and without orders, for the victory over the Spanish off Cape St. Vincent (1797). He was made a rear admiral by seniority and was created a knight of the Bath. In the unsuccessful British attempt (1797) to capture Santa Cruz de Tenerife, Nelson lost his right arm and was returned to England.
Upon his return to service, he was sent on detached duty to find the French fleet. After a long pursuit the French fleet was destroyed in 1798 at Aboukir (the modern Abu Qir), stranding Napoleon I and the entire French army in Egypt. Nelson was showered with rewards and honors, but received only the comparatively modest title of Baron Nelson of the Nile. He was placed in command of a squadron assisting the kingdom of the Two Sicilies. Here he fell in love with Emma, Lady Hamilton, the wife of the British ambassador, who became his mistress.
After the French took possession of Naples (1799) and set up the Parthenopean Republic, Nelson blockaded the city. During his absence on one occasion, the royalist commander, Fabrizio Ruffo, made a generous peace with the Neapolitan republicans. But Nelson, on his return, annulled the treaty and executed the Neapolitan admiral, Francesco Caracciolo, for desertion to the French. When the British commander in chief in the Mediterranean ordered him to Minorca, Nelson refused to obey on the grounds that his presence in Naples was politically necessary, but it was suspected that he did not wish to leave Lady Hamilton.
In 1800 he returned to England with the Hamiltons and soon separated (1801) from his wife. The same year, Lady Hamilton bore him a daughter, Horatia. Nelson contrived his appointment as second in command, under Sir Hyde Parker, of the fleet sent against the armed neutrality of the Baltic powers. He defeated (1801) the Danes at Copenhagen, ignoring Parker's order to cease action by putting his telescope to his blind eye and saying that he could not see the signal. He was made a viscount, returned to England, and was given command of the Channel fleet to repel an expected French invasion. During the interlude of peace (1802–3), he lived in the country with the Hamiltons.