Coordinated as part of an attack on France by the nations of the Third Coalition against Napoleon by William Pitt the Younger and Robert Stewart, Viscount Castlereagh, planning began for an invasion of French territories in July 1805.

Hanover, previously a British possession, was chosen as the goal of the expedition, with Swedish and Russian forces under Gustav IV Adolf and Count Pyotr Aleksandrovich Tolstoy brought in to support the endeavour.

[5] In around October 1805 Napoleon's planned invasion of the United Kingdom was called off and the Grande Armée left its encampments at Boulogne to march towards the Russians and Austrians.

[4][7] The British Prime Minister, William Pitt the Younger, and his Secretary of State for War and the Colonies Robert Stewart, Viscount Castlereagh, were strong supporters of the enterprise, having championed it from as early as July.

They envisaged an amphibious army that could be landed at points across Napoleonic Europe, making "pinprick" attacks against enemy targets while avoiding large battles with the French that could result in "crippling defeat".

[4][11][12] While the original plan had expected the creation of multiple divisions, Don's force was instead made up of two cavalry and six infantry regiments of the King's German Legion (KGL) which were controlled by Brigadier-General Friedrich von der Decken,[Note 1] a brigade of Foot Guards under Major-General Edward Finch, and a brigade of line infantry under Major-General Edward Paget.

[4][14][15][13] Von der Decken planned the operation, ensuring that the force would be transported quickly to avoid the coming of the harsh northern winter that would freeze the ports and rivers necessary in disembarking the troops.

[7] It was initially planned that Don would go ahead of his force to ensure that they would receive a positive welcome upon their landing, but this duty was instead taken by the politician Dudley Ryder, 1st Earl of Harrowby on 25 October.

[21][9] The crossing was difficult, and at least five ships did not complete it, spending seventeen days at sea before returning to Harwich with a portion of the KGL cavalry still on board.

A Swedish force of 10,000 men paid for by Britain was poised to attack from Stralsund, and they were joined by 20,000 Russians under Lieutenant-General Pyotr Aleksandrovich Tolstoy.

[11] The British were not the first part of the Third Coalition to enter Hanover, as a Russian force had earlier arrived to blockade Hamelin, still garrisoned by the French and the only position left to them.

[21][7] Despite this Don's army received a warm welcome, and they quickly secured lines of supplies, while the KGL took advantage of being back in Germany to increase their numbers with local recruits.

[23] The neutral Prussian Army had also entered French-controlled lands, south of Hanover, commanded by the Duke of Brunswick, but quickly looked for their forces to be replaced by those of the coalition.

[21][14][25][30] Cathcart, who had been preparing to serve as ambassador-extraordinary to Russia and Prussia, was titled "Commander-in-Chief of British Forces in Northern Europe", and was given control over Russian, Swedish, or Prussian troops as well.

Despite this aggressive step, Cathcart was also warned to take caution in his warlike manoeuvres; even if he put his army under the command of another force, such as that of Gustav IV Adolf of Sweden at Stralsund, he was allowed to refuse action if he deemed it too dangerous.

[21][31] He began to prepare for the upcoming campaign, discussing how to communicate if the rivers of Heligoland should freeze over, considering whether to advance on Holland or the Lower Rhine, and bringing his army together around the Weser.

[23] This injection of new troops was not the boon it might have been for Cathcart, because on top of the decaying strategic situation, the troopships had sailed through rough weather in the North Sea.

Hardly any of the battalions embarked succeeded in reaching Cathcart whole; the troopship Ariadne, carrying the headquarters party and 300 men of the 9th Regiment of Foot, was wrecked near Calais and all on board were taken as prisoners of war.

[41] Only the 28th and 36th Regiments of Foot succeeded in arriving substantially intact, but large portions of Cathcart's reinforcements were so badly depleted that they were incapable of further operations, with over 1,000 people having been killed.

[40] Pitt's health was severely declining and he had retired to Bath; with the prime minister unavailable, the Cabinet began to send troopships to Cathcart in preparation for an evacuation.

[46][48][49] With Prussia now advancing to occupy Hanover with 50,000 men and with rumours growing that a French force of three divisions was preparing to attack from Holland, orders were sent for Cathcart to evacuate on 19 January.

His army by this time had grown to 26,643 men because of recruitment by the KGL which resulted in it returning to Britain with more battalions and depots than it had left with;[Note 7] only around 14,000 were British infantry.

[57] Positively, however, Hanover had shown that the navy could transport large groups of soldiers across seas at short notice, and in relatively good order barring bad weather.

Pitt died on 23 January, having been further weakened by the knowledge that Britain was alone in war against France, her allies defeated and the British once more pushed from the continent.

[1] Pitt and Castlereagh had reacted quickly to the opportunity to attack Germany, and Atkinson says that the Austrians and Russians made enough mistakes that Napoleon was able to take advantage and defeat them; without them Cathcart's army could do, and did, nothing.

However, he agrees that the plan behind the expedition was sound in theory, its execution being let down by the rulers of the allied nations; while complimentary to Pitt's attempts to strike a blow against France, he is derisory of the other leaders, calling those of Russia and Sweden "insane", that of Prussia "contemptible", and Austria "weak".