This was not a new concept. The Americans had been successful using commerce raiding against British merchant ships during the American Revolutionary War. Outgunned by the Union Navy they realized building lighter and faster cruisers would allow them to outrun the larger warships and go after the slower and usually unarmed merchant ships. Because the South had little shipbuilding capacity, they had these ships built in Great Britain. This was technically illegal, since Great Britain was a neutral country. So these ships had to be built in secret.