The most exhaustive -- not to say wearisome -- examination of Napoleon's invasion projects is undoubtedly that of Edouard Desbriere, a major in the Historical Section of the French General Staff at the turn of the century. Of the five fat tomes comprising his Projects et Tentatives de Debarquement aux Iles Britanniques, the last three, plus yet another later one on the Trafalgar campaign, deal with Napoleon's invasion project.

The conclusion Desbriere reached from this mountain of research was that Napoleon did not mean to invade at all. But it is important to realise that Desbriere's work formed part of a comprehensive analysis of the Master's achievement by the General Staff' at a time when both it and the Ecole de Guerre were infected with a bad case of Napoleon- worship -- an enthusiasm, incidentally, that would eventually have to be paid for in the lengthy casualty lists of 1914.

However, to his horror, the Major found his researches leading him to the unthinkable - that the invasion project was a sorry welter of administrative confusion. To Desbriere, there could be only one explanation for this farrago of Napoleonic ineptitude - to take at face value the Emperor's statements that, all along, the project was merely a cover for the strike eastward into Germany that took place in late 1805.

But Desbriere's conclusion really bears on the operation's feasibility rather than the genuineness or otherwise of Napoleon's intentions. The wretched Berthier may well have still been juggling his manpower figures right up to the last minute, but even if'we have to admit the evidence, it does not necessarily signify; Napoleon was never exactly noted for the smooth efficiency of his staff work. But to subscribe to the idea that Napoleon should devote so much time and effort to a mere subterfuge, that he should risk the bulk of his naval strength in a dangerous foray to the Caribbean and back simply as a diversion, flies in the face of common sense.

Personally, since first reading the relevant volumes of the Correspondance, I have never really doubted his commitment to invasion right up to August 1805; the torrent of edicts and messages contained therein give off the authentic, powerful whiff of remorseless intention.

An amphibious offensive required three things: an expeditionary force, a means of transport and harbours for assembly and embarkation. From the early summer of 1803, all had to be created more or less from scratch. Hardly anything of the previous invasion flotilla of' 1801 proved worth salvaging. Thus a nation-wide construction programme was initiated in every available harbour and inlet from Holland to the Spanish border, with a final production target of 1,000 supplemented by a transport flotilla of some 940 hired commercial boats plus a Dutch flotilla of 260.

Strangely, considering that French naval architecture was the best in Europe, the custom-built boats were little better than tubs, each of' the four types having some elementary design fault or another, all embarrassingly revealed to the Emperor himself on the occasions when he ordered them out of harbour to face the Channel elements.

At least in the case of the army it was more a matter of reorganisation, but this in itself was a huge undertaking; the numbers were there but they were still grouped in what were still, in essence, the several ad hoc armies of the Republic.

In June 1803, Napoleon began issuing a masterly series of movement orders that set virtually every unit in France on the road, some to the coast to rendezvous with newly-built boats before they sailed for the assembly ports, others to various camps on the Channel coast. By the end of the year, 70,000 men were in place under the commands of Davout, Soult and Ney. There they were set to practising beachlanding, and pick-and-shovel work on harbour extensions.

Napoleon eventually settled on four harbours, all on the reach of Channel coast that trends south from Cape Griz Nez. Boulogne was by far the most extensive in terms of eventual capacity and troop concentration and was, in effect, Napoleon's centre of operations; he spent most of his sojourns on the Channel coast there and had a cliff-top eyrie built on the Tour d'Odre from which he liked to survey the scene below. All the designated ports - Boulogne, Etaples, Ambleteuse and Wimereux - were tiny tidal harbours, dry at low tide, that needed large- scale extensions to fulfil their intended function. Outlet channels had to be straightened and deepened, and basins excavated to take the multitude of boats.

Furthermore all the ports - indeed the entire sector - had to be protected against enemy attack, a hazard Napoleon took most seriously. Such was the eventual density of artillery cover that it baffled all the Royal Navy's plans for attack, including Robert Fulton's 'torpedoes'. But Boulogne presented a particular problem, the implication of which forced Napoleon towards the key problem that eventually had to be faced by all would-be cross Channel invaders.

For all the work done, it was found that only a maximum of 200 boats could clear the harbour entrance in a single tide. This meant that no fewer than six tides - 72 hours - was required to get the entire proposed Boulogne contingent out to sea. Hitherto, Napoleon's intention had been to make the crossing by stealth, under cover of darkness or fog. It was a typically uninformed landlubber's idea and, to their discredit, his senior naval commanders either sycophantically endorsed it or lacked the nerve to challenge it.

But to mount six lifts over six days undeniably demanded wresting temporary control of the Channel from the Royal Navy. Unfortunately, the latter had a long-established, recently perfected strategy for coping with just such an eventuality. It comprised a close all- weather blockade of Brest by the Channel fleet, together with a string of squadrons posted along the Channel as far as the Texel.

This is why, on the British side, the Navy were always the leading sceptics about the chances of a successful invasion by what they scornfully dubbed the 'mosquito fleet'. Their scepticism was well-informed; inshore squadrons kept continuous watch on enemy preparations and regularly infiltrated agents on to the French coast. But the government, like all others before and since, could not afford to take the threat less than 100% seriously.

Hence the Home Front of 1803-5 is in many respects irresistibly evocative of that of 1940. Though the opinion-formers of the earlier period lacked the mass media available to the latter - radio and cinema - they nevertheless conducted a vigorous campaign with the means to hand, which was print. The ,nation was deluged with cartoons, addresses, songs and verse churned out by the London booksellers for the benefit of the lower orders who, it was feared, might not see completely eye-to-eye with their betters over the prospect of Boney in residence at St. James's.

Whether it was at least partly due to this avalanche of morale-boosting propaganda, some half-million or so Britons of a total population of about nine million turned out to do their bit for king and country in one martial capacity or another. Some of course had no choice: these were the 50,000 or so Regulars. But a large proportion of the Army was scattered in overseas garrisons and, in the case of Ireland, holding down a mutinous populace.

To supplement the Regulars the Militia, brought into being some 40 years previously, added a further 70,000 or so to the nation's defences, though the calibre of both officers and men was, to say the least, suspect. Miraculously, another 50,000 were conjured from nothing by a shrewdly devised scheme to provide a second battalion for 50 line regiments, collectively known as the Aray Of Reserve. But most Britons were shouldered a musket during this period, in all some 380,000, did so as members of Volunteer units, the Home Guard of the day. The Volunteers were very much civilians-in- arms, part-time soldiers who drilled on Sunday mornings after church or in the long summer evenings on square or green across the land.

The hierarchies of these corps closely reflected those in society at large, with officers that were representative of the aristocracy and gentry in the shires and the prosperous professional and business class in the towns. But while these men had the weight of social position behind them, they could not call on the savage Regular disciplinary code, and often had to rely on the ploys of what we would nowadays call man-management.

I have come across several instances of indiscipline and mutiny in Volunteer corps, often going unpunished, and it would be interesting to know if there were many more. Much still remains to be discovered about this subject, and I would recommend it to members seeking an unhackneyed research field. The fascinating but mind-boggling ramifications of four separate recruiting campaigns running simultaneously throughout this period are incapable of succinct summary. I refer those interested to the relevant chapter of my book.

No charismatic Churchillian figure was thrown up by the crisis of 1803-5. Indeed the Prime Minister of the day, Henry Addington, was widely regarded as a mediocrity who was not up to the job. His brief successor, William Pitt, was worn out and well past his peak and, in any case, unlike his father, Chatham, had never been the stuff of which popular national leaders are made. Nevertheless the nation's defences were in safe hands: at the Admiralty Lord St. Vincent and Lord Barham, and the Horse Guards Frederick, Duke of York.

Though the Duke of York's career in the field left something to be desired, the crisis amply demonstrates his credentials as a defence planner. His scheme rested on one ineluctable and uncomfortable fact of geography - the short distance between the south-east coast and the capital; what he called 'this original defect in our position'. He was well aware that his troops could not compare in quality with Napoleon's legions.

The only available equaliser was quantity and his aim was to maximise its advantages. But the problem was that, if the balloon went up, the Volunteers would be scattered across the length and breadth of the realm in town, village and farmstead. The solution was a carefully timed mobilisation plan, triggered by an alarm which would be transmitted across the Strait by the Navy and then carried to London by the new-fangled telegraph. Mobilisation would then be successively activated by relays of mounted messengers.

In the meantime, the 40,000 Regulars Frederick had already managed to pack into the south-eastern counties would meet the first assault waves right on the beaches before they had a chance to sort themselves out. If the invader nevertheless managed to gain a lodgement, mobilisation would continue to roll, channelling ever greater hordes from ever remoter regions into London, where they would be deployed as the situation required.

In order to impede the enemy advance, the Duke had, since the renewal of hostilities, vigorously campaigned for a programme ofjudiciously-placed fortifications. Due to the obtuse stone-walling of Pitt's elder brother Chatham in his capacity as Master General of the Ordnance Board, a frustrated Frederick in fact achieved less than he wanted. But at least all was in place for a fortified enceinte around the capital and the flooding of the Lea valley, and the old earthwork defences of Chatham Yard had been put in good order.

But of Frederick's three pet projects for coastal defence, the fortification of Dover's Western Heights, the Martello towers and the Royal Military Canal to seal off Romney Marsh, there was precious little to be seen by the crisis month of August 1805: a single earthen rampart on the Western Heights, a halfcompleted excavation on Romney Marsh and not a single brick laid on the 74 designated Martello sites.

Britain's defence planners did as much as they could to meet the threat of 1803-5. If they failed, they still had contingency plans ready for a last-ditch stand. Martial law proclamations had been run off and preparations made for appointing local area controllers and paying troops in coin. They royal family, together with the Bank of England's deposits, were to be evacuated to Worcester and the entire contents of Woolwich arsenal transported by canal into the Midlands.

The abiding fascination of the three great invasion projects against these islands - the Armada, Napoleon's and Hitler's - is that they lead us down avenues that professional historians sternly label 'off-limits', into the realm of historical might-have-been or 'alternative history'.

What if Britain had been Catholic these last three centuries? Suppose we had all been living for the past half-century under the heel of a Thousand-Year Reich? And imagine a Britain Gallicised for nearly 200 years: no Waterloo, no endless wranglings over the ERM, no 'Sun' headlines like 'Up Yours, Delors'. It makes you think, doesn't it?