Abraham Lincoln and Union leaders realized from the war's outset the grave threat British intervention posed.  Intervention likely meant successful Confederate independence.  No matter what form, be it mediation, recognition, or literal intervention, any attempt by the British to interfere was based upon separation of North and South.  The causes of the Union and Confederacy were mutually exclusive; either the Union remained whole or the Confederacy earned independence.  British intervention effectively destroyed the cause of preserving the Union.