Claude R. Wickard, Food Administrator for the United States briefly imposed a ban on sliced bread as a wartime conservation tactic in 1943. People in the US were restricted from buying and using a number of things to conserve materials for the World War. This was just supposed to be another one of them.
According to the New York Times, officials explained that "the ready-sliced loaf must have a heavier wrapping than an unsliced one if it is not to dry out." It was also intended to counteract a rise in the price of bread, caused by the Office of Price Administration's authorization of a ten percent increase in flour prices.
However, public protest was pretty high on this, and on March 8, 1943, the ban was rescinded. Wickard stated that "Our experience with the order, however, leads us to believe that the savings are not as much as we expected, and the War Production Board tells us that sufficient wax paper to wrap sliced bread for four months is in the hands of paper processor and the baking industry.