The effectiveness of the Southern Strategy in Charleston can be assessed through the events surrounding the British seizure of the city. The most direct outcome of the Southern Strategy in this context is captured in the response: Charleston was seized by the British, and 5,000 men surrendered.
This event was significant as it represented a major defeat for the Continental forces and a strategic gain for the British, allowing them to control a key city in the South and disrupt American supply lines. The surrender of such a large number of troops was a blow to the American cause at the time, demonstrating that the Southern Strategy, employed by the British, had effectively diminished American control and morale in that region.
While other answers point to instances of resistance or the potential for future successes, the surrender in Charleston stands out as a clear manifestation of the effectiveness of British strategy at that time.