This question has always confused me and here’s why. Napoleon’s greatest threat was by far the Kingdom of Great Britain (and later the United Kingdom). While he may have had superiority on land, Napoleon knew that he would be defeated at sea if he tried to cross the English Channel but he may have had an advantage.
First off- Ireland. While France may have intervened in the 1798 Irish Rebellion, they never made an impact like they did in the American Revolution. However if Napoleon was smart he could’ve instigated a new revolt in Ireland to steer as many British troops away from southern England as he could. Another, and in my opinion the best, option for Napoleon could’ve been Scotland. Years prior the Glorious Revolution occurred in which the House of Stuart was overthrown as the rulers of Scotland & England and eventually the throne fell to the House of Hanover (which ruled during the Napoleonic Wars). If I recall correctly, Scotland was especially loyal to the House of Stuart even after their fall.
Napoleon must’ve known that if he was to conquer Britain he couldn’t rule as its king so he could’ve simply used the last member of the House of Stuart at the time- Henry Benedict Stuart a cardinal from Rome who was also the brother of Charles Edward Stuart who led the unsuccessful Jacobite Rising of 1745. If Napoleon had backed Henry then many in Scotland and in Ireland would’ve supported his claim to the throne.
Would this have been a feasible plan? If so, why did Napoleon not attempt this? Even if the only threat to him was the English navy, England would’ve been surrounded by enemies.
John Jervis, 1st Earl of St. Vincent, said it best in 1801 in a letter to the Admiralty: "I do not say, my Lords, that the French will not come. I say only they will not come by sea."
Britain had absolute naval supremacy over the French fleet in the period of the Napoleonic wars; without superiority at sea, which the French could only hope to gain by destroying the British fleet, the French had no hope of an invasion of Britain. The best they could hope for was a repeat of the raid they attempted to stage at Bantry Bay in 1796, which accomplished nothing but the destruction of most of their invasion fleet in storms.
The British were, of course, aware of the invasion threat and attempted to ameliorate it by keeping the French fleet blockaded and engaging it whenever it came out. I wrote about blockade more here. The major French effort under Napoleon to draw out and destroy the British fleet was the campaign that culminated with the battle of Trafalgar, where the British forces destroyed the combined Franco-Spanish fleet. I wrote about Trafalgar here.
The prose gets a little purple, but Mahan was correct when he wrote of the British fleet that "those far distant, storm-beaten ships upon which the Grand Army never looked, stood between it and the dominion of the world."