Harry Harrison 
Stars and Stripes in Peril 
Stars and Stripes Book 2

An uneasy truce is broken by the British, who refuse to believe that the might of the British Empire can be bested by their former colony. They begin building a road across the Isthmus of Tehuantepec, where Mexico is at its narrowest. This will enable them to bring in colonial troops from the Pacific Ocean to the Atlantic to attack the United States along the undefended Gulf coast.When the Americans attack the road, the war bogs down in futile jungle warfare. Only a brilliant plan by General Robert E. Lee enables the United States to broaden the war and attack the British forces where they least expect it – very close to home.