Breakfast hash has been around for centuries and is still enjoyed today, but it was particularly popular during World War II, when meat rationing required home cooks to get creative.
Traditional home-cooked hash was made of “chopped cooked meat” and cooked vegetables (usually potatoes and onions) mixed with broth and fried on the stove.
While it was served at lunch and dinner as well, hash for breakfast was an economical way to use up dinner leftovers and stretch those precious quantities of rationed meat by adding flavorful fillers.
In wartime ads, Armour and Company, the first company to produce canned meat, reminded Americans that “our war needs make it vital now to save every bit of food left over,” and offered a free booklet called “69 Meat Ration Recipes” that included breakfast hash recipes using a variety of fresh and canned meats.
Source: http://www.Historyfacts.com