The Washington correspondent of the Ballimore American, n pretty good authority, speaks of the following as the plan of operaÃ¼ons in slavery's war, now entertained by Mr. Polk : "The idea seems to be tofortify a line of country which will connect the Rio Grnnde with the Pacific, secure us in the possession of the ports of Mexico on the Gulf, and all the important Mexican ports on the Pacific, and thus secured, to colonize Norihern Mexico, California, New Mexico, Chihuahua, and parts of Sonora and Tamaulipas. os fast as we may. This done, the advice is to halt until Mexico sues fur peace. The invasiÃ³n of her capital would cost a hundred thousand men, wilh the necessary troops to secure the important points on the road. Todo what is proposed will probably require a regular and volunteer forcÃ© of 25,000 or 30,000.