It was especially important to keep the men busy when at the American coast due to the great temptation they had to desert. Once, an entire group deserted when sent out in a ship's shore-boat, and they sold the boat for $50! America was the perfect place to defect to because;
With Napoleon's defeat, more British soldiers became available. Britain's top officer for eastern America was Admiral Alexander Cochrane. His fleet brought four regiments of soldiers (a regiment has about 800 men) from France to America's Chesapeak Bay. There they met with the British officer that knew most about the Chesapeake area; Admiral George Cockburn (pronounced Coburn).
The soldiers, a land matter, were under the authority of Major General Robert Ross, who had gained lots of military experience fighting the French in Spain. He was to work with Cockburn in effecting the orders of Cochrane; to invade and burn Washington, and to destroy Baltimore.
Washington had little military value. But when Americans burned the city of York in Canada, it made the British want revenge. They felt it would ruin morale and cause America to end the war. Baltimore, on the other hand, was a logical military target. The British considered it a "nest of pirates", since most US privateer vessles came from Baltimore.
As for revenge, the British might have considered it settled after their deplorable actions in Hampton Roads, VA the previous year. There, it was mostly a certain large group of soldiers (non-British but serving in the British army) that committed murders, lootings, rapes, and just about any violent criminal act one can think of.
A large aspect of the attack on the Washington area was simply to get the US to keep more soldiers there and stop sending so many to Canada where land battles were getting serious. Ever since the war began, battles in Canada had usually been British victories. Most were small affairs. The summer of 1814 was different. The US had sent not only large numbers of well supplied troops, but sent new, young, competant and aggressive leaders. The battles of Lundy's Lane and Ft. Erie were especially significant. Ft. Erie was a great US morale booster, and Lundy's Lane, while mostly a stand-off, was the most vicious and deadly battle of the war so far. If the US gained any ground in Canada, peace would be hard to reach. The reader might suspect perhaps the British were taking too indirect an approach to this problem, and should have sent the Washington Invasion forces directly to Canada. They HAD sent several new regiments to Canada, and the land could only support so many soldiers.
It might be said the British never really thought they'd reach Washington, but just wanted to cause trouble in the area to divert US soldiers from battles in Canada. Oddly, soldiers and supplies CONTINUED being sent to Canada in the summer of 1814. There, in July, were the Battles of Lundy's Lane and Ft. Erie; the most vicious fighting of the war. Lundy's Lane was a stand-off, but Ft. Erie was a victory for the US that greatly improved military morale across the nation. But is a victory up there worth risking a defeat in the Nation's Capitol? It was expected we would have victory in BOTH locations.
The British had a land force of about 4500 soldiers which included some marines and sailors. This was a significant force in those days, but not huge. Admiral Cochrane felt it was not really enough to go as far inland as Washington was, but Cockburn assured him it was plenty. Ross was confident that his men would do well, but knew little about Americans or America. Therefore, Cockburn was the most informed person and took the lead in the adventure.
There were many large ships that carried all the British soldiers and supplies, and when they came near the Maryland shore, it was no secret. It was expected that they'd be coming after Napoleon was defeated. It was just a matter of when. The sails appeared to Americans at Maryland's Point Lookout on August 17. At once, the Maryland coast, Baltimore, and Washington City were all in an uproar.