Viscount Castlereagh was Secretary of State for War between July 1805 and September 1809, except for the fourteen months of the Ministry of All the Talents administration, between January 1806 and March 1807, during which time the office was occupied by William Windham. Shortly after Castlereagh's first appointment in 1805, and prior to Trafalgar, the French military camps at Boulogne were broken up and the troops marched eastwards, removing the immediate threat of invasion. This was a period of great uncertainty regarding the loyalties of the states of Europe, but on 9 October the Third Coalition was established, uniting Britain, Austria and Russia. Prussia refused to join, preferring instead to form an alliance with France in exchange for the possession of Hanover. Napoleon reneged on the deal so no alliance was concluded, but France did transfer some of its force stationed in Hanover to Bavaria. In light of this, Britain prepared to send a light corps of about 10,000 men, by way of the Elbe, to reoccupy Hanover.

Whilst planning this campaign, Castlereagh had to resolve a conundrum that immensely frustrated army commanders: the consistent shortage of horses for their cavalry battalions, for the Commissariat, which required the animals for hauling supply wagons, and for the artillery divisions, to haul the heavy guns. This was a perennial challenge because of the volume of tonnage required for transporting each horse and rider, for forage and supplies, and for artillery equipment and wagons. There was a shortage of suitable ships and the costs were significant: £8–£10 per horse per month compared to troop ships at just under £2 per man per month.

The Expedition to Northern Germany, 1805

Lieutenant-General George Don was appointed to lead the vanguard of 10,000 men, including 5,000 troops of the German Hanoverian legions who were then in Ireland. Admiral Lord Keith was the naval commander while Lieutenant- General Lord Cathcart was appointed Commander-in-Chief of the army. The scope of the operation matured into a quest to drive the French from Holland; the 10,000 men from England would be joined by a further 10,000 and also by 20,000 Russians, 10,000 Swedes and 26,000 Danes to create a force of 70,000– 80,000, possibly even 100,000 men.