The Continental victory at Saratoga in 1777 and the Treaty with the French in 1778 transformed the war. The British were immediately faced with a global conflict with France, and as a result, they changed their strategy. Rather than mounting a full-scale military campaign against the Continental Army, the British decided to focus efforts on rallying the loyalists, who they believed were the majority of the American population.
The British turned their efforts to the South, where they thought the loyalists were strongest and they hoped to enlist slaves to their cause. But the British had overestimated loyalist sentiment in the South. British military presence actually forced many Southerners who had been sitting out the war to take sides, usually in favor of the Patriots.
The British were successful in most conventional battles in the South. They occupied Savannah, Georgia, in late 1778 and Charleston, South Carolina, in May 1779. However, the Continental Army under the leadership of Generals Nathanael Greene and Daniel Morgan started using guerrilla and hit-and-run warfare. At the same time, the British underestimated the logistical problems they would encounter, especially when their army was in the interior away from the supplies offered by their fleet. Patriot forces, on the other hand, were supplied and could hide among the local population. As a result, the British southern strategy was a dismal failure.
Source: Revolutionary War: Southern Phase (1778-1781)
Library of Congress