Prévost laid out his plans to George Germain, Secretary of State for America, in a letter dated January 18, 1779:
“...I shall immediately proceed up the river to see what advantages can be expected from the frontier inhabitants of Carolina who give the strongest hopes of joining heartily whenever they find that they are to be supported. Should it appear that they do not mean to take the active part which they have promised, I shall confine my plan of operations to harassing the enemy by excursions and effectually securing this province, to which all the loyal subjects who have fled to Florida for shelter will soon return if they can be assured of being protected. If, on the contrary, the frontier inhabitants should evince the zeal which they profess, I cannot doubt but great advantages may be delivered from a diversion on their side supported by an attack at Beaufort or any of the settlements on the coast of South Carolina…”
In other words, Prévost intended to drive into the Georgian backcountry in order to galvanize American Loyalists, while at the same time launching a diversionary attack on the South Carolina post.
To effect this plan, Prévost made his headquarters at Ebenezer, Georgia, a town 25 miles northwest of Savannah, and roughly opposite Purrysburg, South Carolina, where Major-General Benjamin Lincoln was assembling a large American army.
Strategic Situation in the South (click to enlarge). This map shows places referred to in this blog post that were occupied by British and American forces in January, 1779. The British held Savannah and Ebenezer in Georgia [red dots], while the Americans held Augusta, Georgia, and Purrysburg, Beaufort (on Port Royal Island), and Charleston, South Carolina [blue dots]. Based on a 1776 map in the David Rumsey collection.
On January 24, Prévost sent Lieutenant-Colonel Campbell with a large force  to take Augusta, and sent a diversion against the South Carolina coast. In his words:
“In order to facilitate further the success of Colonel Campbell’s operations... I sent three companies of light infantry that came from Florida to go up Beaufort River under the escort of the Vigilant, and to draw the enemy’s spies about us to conceive that more troops were sent by sending a regiment of Hessians to the place of embarkation two miles from Savannah and marching them in the night back to their barracks.”
The three companies of light infantry belonged to the 16th regiment, and the 3rd and 4th battalions of the 60th regiment. This small force, no more than 160 men, was commanded by Major Valentine Gardiner. Nevertheless, their arrival on the South Carolina coast put the Americans in a state of alarm.
Colonel Charles Cotesworth Pinckney described the popular feeling in Charleston, South Carolina, in a letter to Brigadier-General William Moultrie:
“...many people think, this movement of the enemy is to post themselves at Port-Royal, and there wait reinforcements from the northward; others, that it was done only to cause a division and to weaken your little army, that they might more easily pass the river [i.e., so that Prévost could cross the Savannah River and invade South Carolina]; others, that it was to destroy the town and fort, plunder and return to Tybee [an island near Savannah]; but some with more penetrating looks and significant nods; that the vessels appearing in scull creek, as if intended, (Port-Royal was, or is, only a feint,) to cover a real design of landing suddenly on the Euhaws, march to a pass of consequence near Elliott's hill, on the southern road, and there throw up some field works, which with a few cannon will entirely cut off the communication from town to Gen. Lincoln's army, and put him between two fires; this last manoevre, is thought of so much consequence to the public safety, as to raise the public anxiety...”
Lincoln was not deceived, and he remained at Purrysburg with his army. However, Lincoln also did not want to see a key fort on Port-Royal Island fall to the British. He therefore ordered General Moultrie to take control of South Carolina militia on and near Port-Royal Island and defend that threatened post.
1. Campbell described the composition of this force in a March 4, 1779, letter to Lieutenant-General Henry Clinton:
“A proper supply of provisions having been collected at Ebenezer, I marched on the 24th January with Sir James Baird’s light infantry [a light company of the 71st regiment, with, perhaps, a corps of Provincial light infantry attached], the 1st battalion of the 71st regiment, the New York Volunteers, one troop of light dragoons [commanded by Captain Tawes], Colonel [Alexander] Innes’s Carolina Loyalists [i.e., the South Carolina Royalists], and the Florida Rangers, making in all one thousand effectives, rank and file, with 4 field-pieces, one howitzer and two royal mortars.”