The comforting flavor of grilled cheese with tomato soup is another unexpectedly modern creation. The nostalgic dish only first emerged in the early 20th century, with the period between the Great Depression and post-World War II giving rise to the dish. Toasted cheese sandwiches were popular in the early 20th century, both as an economical recession meal and a popular military ration. Tomato soup followed, and the convenient and affordable combo caught on with bureaucratic institutions, cafeterias in schools, and governmental organizations. In addition to delectably complementing the taste and texture of the sandwich, the tomato soup also became valued for its vitamin C content. And so, the popular combination integrated into American cuisine, and has remained prominent ever since.

Source: Chiwhound