Why did Britain change its strategy to focus on the Southern colonies in the summer of 1778?

The French allied with the colonials, and the British believe they had supporters in the south.

Explanation:

The French signed a treaty with the colonials in 1778 which forced the British to rethink their tactics.  They were now having to fight both the newly formed Americans and the French.  The British falsely believed they had more colonial supporters in the south, so began to focus their efforts there in attempt to subvert the French involvement.  This lead to the colonials fighting the British in the south with guerrilla warfare tactics instead of proper field combat, as the British were use to.  For America, the French helped a ton when they joined the war (Thanks Benjamin Franklin).

from the conditions of the working class in england by friedrich engels,the main idea is :

herodotus taught this. herodotus was an ancient greek historian who was born in halicarnassus in the persian empire. he is known for having written the book the histories, a detailed record of his "inquiry" on the origins of the greco-persian wars.