Why did the South prove to be more difficult to conquer than the British expected?

There were multiple reasons, and most of them were really just because the British failed to take control. (More in the explanation.)

Explanation:

The British assumed that they had loyalty among their numbers, traitors were active. The British failure to deploy sufficient forces to take control of the South.

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explanation:

2018 means 2018 ce or common era

there have been other eras too including bce, which included way more years.