From the U.S. Army's Airborne
School at Fort Benning, Georgia.
To make a soup
sandwich you get some bread, then take a big ladle of soup, pour it on the bread, take another piece of bread, put it on top, and try to eat it.
Of cource it falls apart almost immediately.
In the Military
a person who is a 'soup sandwich' could be any combination of disorganized, late, unprepared, or unmotivated