Why Nelson should have deployed his good eye

When Horatio Nelson put the telescope to his blind eye and exclaimed "I really do not see the signal" (to break off action), he spoke more truly than he realised. For the battle of Copenhagen, fought on April 2 1801 and aimed at breaking the armed neutrality of the northern powers, may now be seen as a tragic error caused by a failure of intelligence.

It is a pity that Steven Maffeo, a young American academic in charge of a reserve US intelligence unit, did not have access to Elizabeth Sparrow's researches. She reveals in her recent book, Secret Service , a fatal, unexplained delay in communicating to the British Baltic fleet the news that the mad Tsar Paul of Russia had been assassinated a fortnight earlier. He had been the linchpin of a pro-French alliance that threatened to cut off the Royal Navy from its principal source of war matériel .

Sparrow makes a credible case for involvement in the coup by the precursors of MI6. The sluggishness of Admiral Sir Hyde Parker's fleet in assuming its battle station at Copenhagen was due not so much to faint-hearted ships' pilots unwilling to navigate shoal waters, or a dockyard workers' strike - much less by the elderly admiral's dallying with his new teenage bride - but to diplomatic manoeuvring to detach Denmark from the Russian embrace. Nelson, who as Maffeo points out, had been expecting to fight the Russians, thought that keeping the fleet "out of sight is to seduce Denmark into war... I hate your pen-and-ink men; a fleet of British ships of war are the best negotiators in Europe".

Perhaps the real reason for Parker's raising the famous recall signal to Nelson was found by Sparrow in the Public Record Office. His flagship's log tells of the last-minute arrival of a mysterious boat with news so urgent that he immediately dispatched his chief of staff to his second in command calling off the attack.

Nelson was already in the thick of what he regarded as his hardest-won battle. He had found the blind spot in the Danish defences the same way he had outmanoeuvred the French at the Nile, by secretly plumbing the depth of Copenhagen harbour. Only Nelson could have snatched victory from the mouth of disaster, for as Maffeo demonstrates, the Copenhagen expedition was compromised at the outset, with The Times disclosing: "The public will learn with great satisfaction, that Lord Nelson is about to be employed on a secret expedition."

Maffeo has made a valuable contribution to our understanding of a period of naval history that is in danger of being dominated by the genius of Nelson, particularly in this bicentenary of his victories.

Precursors of Nelson is a timely reminder by 15 specialists in 18th-century maritime history of what the admiral owed his predecessors. Richard Harding describes Edward Vernon's aggressive "blue water" policy, emulating Drake by taking the fight into the ports of the Spanish Main, with his seamen fired by a new spirit of professionalism rather than the neat rum that had given them the DTs and that he ordered to be watered to keep them fit for action. George Anson receives his due as architect of the reformed Georgian navy, introducing the concept of a Western Squadron.

Nelson's first tutor in battle tactics was a Captain William Locker, whose adage was "lay a Frenchman close, and you will beat him!" Locker had followed Lord Edward Hawke through the reefs of Quiberon Bay in a howling gale and witnessed the utter destruction of a French invasion fleet. It was a classic chase action, with each captain free to use his initiative to come to grips with an enemy.

The rigid line of battle tactics that had so often denied an admiral victory were increasingly being disregarded; Admiral Rodney broke the French line at the Saintes in 1782 and Nelson, a young frigate captain in the Caribbean, learnt the lesson well. When in February 1797 his mentor, Admiral Sir John Jervis (here the subject of an excellent essay by Patricia Crimmin), was in danger of having his prize, the Spanish fleet, slip away from him off Cape St Vincent, Commodore Nelson swung his ship out of line and blocked their retreat, capturing the 100-gun flagship and another monster man-o'-war into the bargain. It was Jervis who gave Nelson the detached command that led to the battle of the Nile (the intelligence duel that Maffeo recounts grippingly).

Two influential but now half-forgotten figures in Nelson's career are rehabilitated here. Charles Middleton, Lord Barham, is the only non-fighting seaman in the book but earns his place as the great administrator who was called "the master mind and director of the whole (Trafalgar) campaign". He was then 79 and could look back on a 40-year career that had prepared the sailing navy for this its finest hour - raising the number of men-o'-war to 90 and ensuring they were copper-bottomed against marine growth and armed with the heavy Carronade.

Sir William Cornwallis was known affectionately to his men as "Billy Blue". His 1804-05 campaign as C-in-C the Channel Fleet was, according to Andrew Lambert, a "masterpiece of naval operational art in the age of sail". Those "far distant, storm-beaten ships" of his close blockade of Brest ensured that Napoleon's fleet dare not sortie for invasion without the support of Villeneuve's southern squadron. He pinned the French to their own coasts, freeing Nelson to settle the score finally at Trafalgar.

John Crossland is a writer specialising in naval history.