At Copenhagen, Nelson used his missing eye as a way to decline an order from his superior to withdraw - however, that message may have been sent in the expectation that Nelson would refuse it if he felt it appropriate, and the story may have been a myth.

In the last years of the 1700s and the early years of the 1800s, tensions were rising between Britain and the major Baltic powers. The British reserved the right to search the vessels of neutral ships, and to seize them and their cargo in the event that they were carrying contraband to a hostile power. Denmark in particular objected to this, with a couple of short actions being fought between British and Danish warships. The Russians also objected to British foreign policy, and imposed an embargo against British shipping. In December 1800, the Baltic powers of Russia, Prussia, Sweden and Denmark signed the Northern Convention. This was an agreement to resist any searches of their ships, by either Britain or France. Britain could not accept this, as the Baltic was a major source of high-quality shipbuilding materials - pitch for sealing seams, hemp for ropes and timber for building ships. With the Baltic powers resisting searches, the British embargo in these materials would be heavily weakened. If these powers chose to retaliate against Britain, then it could cut Britain's supplies of these vital materials.

To counteract this, the Royal Navy formed a squadron for dispatch to the Baltic. It was under the command of Admiral Hyde Parker, with Nelson in second-command. It consisted of nine ships of the line, five of the older, smaller 64-gun ships and a large number of smaller vessels. Parker's orders were to force the passage through to the Baltic, and destroy the Russian naval bases at Reval (now Tallinn) and Kronstadt (near St. Petersburg). If Denmark did not withdraw from the Northern Convention, Parker was to destroy their fleet. This would prevent them acting against British shipping, and stop the French taking any control of it.

The British squadron arrived off the coast of Zealand towards the end of March 1801. An ultimatum, which was refused, was sent into Copenhagen on the 17th March. The main fighting power of the squadron arrived on the 21st March, but several days were needed for some of the smaller ships to arrive. This time was spent discussing plans for an attack on Copenhagen. On the 25th, an attempt was made to pass through the Great Belt, one of the two main straits into the Baltic, but this was called off as too risky. Instead, it was decided to head through the Sound, the other main passage, and fight the Danish fleet off Copenhagen. Several days were needed before the weather was right to pass through the Sound, which was done on the 29th.

The Danes had used this time to prepare for the oncoming battle. They had had anchored a long line of ships in the bay off Copenhagen, covered by forts ashore and floating batteries. There was also an extra line of defence; the waters off Copenhagen were shallow and dominated by shoals. In particular, there was a large shoal, the Middle Ground, between the Danish line and the British fleet. Not every British ship could travel over it or manoeuvre effectively in the deep-water passages around it. Aware of this, Parker chose to divide his force. He would remain in control of the larger or less manoeuvrable ships and remain in reserve, while Nelson commanded the remainder of the force and brought it in to attack.

On the 1st April, preparations began for the attack. The British squadron moored close to the site of the action, and overnight small boats were used to locate the deep channel. Nelson produced a simple plan for the action. The British ships were to sail along the line of Danish ships from the south, overwhelming each one with concentrated firepower. Once the defending ships had been swept out of the way, the British could then bombard Copenhagen and force a resolution.

This plan was set into action at about 10:00 on the 2nd April. It almost immediately ran into trouble. Three of Nelson's ships ran ashore in short order between 10:40 and 11:05. One of these ships, the Agamemnon, was out of range of the Danish line, but the other two were just about within it. The remainder of Nelson's fleet threaded the needle between the two ships that were aground and the Danish ships, and were soon heavily engaged. They faced a difficult challenge - while the Danish ships were largely inferior in size and armament to the British ships, they were well-manned and supported by difficult targets in the forts and floating batteries. However, the weight of British fire, and the experienced British crews began to tell. By 12:45, two of the heaviest Danish ships had been essentially taken out of the action.

This was far from clear to Parker, who was moored about five miles from the action. His view of the fighting was clouded by smoke from the gunfire (a common problem in naval actions at the time). The ships closest to him, a frigate squadron, were suffering heavily from the fire of the main Danish fort, the Trekoner Fort. The only ships of Nelson's main body that could be seen were the ships that were aground. Parker hoisted Signal No.39 - in the RN's code of signals, this meant 'Discontinue the action'. However, he would also dispatch an officer, Captain Otway, to inform Nelson that he could continue the action if he saw fit.

It was in response to Parker's No.39 signal that Nelson is said to have raised his telescope to his blind eye, and uttered the line 'I really do not see the signal'. It's unclear whether this actually happened, and it is quite possible that it is a myth to emphasise the fighting reputation of a national hero. Whether he did it so dramatically, what is clear is that he chose to ignore it and continue to fight. He knew the situation on the ground much better than Parker did, and could tell that his squadron was close to success. His confidence was richly repaid, and by 2pm, the Danish fleet was close to surrender, clearing the way for a final ultimatum to be sent to the Danish government. A truce was agreed at about 3pm