ANSWER-National Security. About 40% of WWII draftees were rejected due to ill heath, bad eyesight, or bad teeth due to malnutrition, a result of the Great Depression amongst other things
http://en.wikipedia.org/wiki/National_School_Lunch_ActEstablishment of the program
The main driving force to the establishment of the program was not what many would think (only health and nutrition issues), but also national security. The driving force behind the program was actually General George C. Marshall, then a minister on the cabinet in the Truman's Government.
During the WWII, the United States facing the needs for an expanding military to cope with war on two fronts. However many of the conscripts, especially those from working class families had had a hard life in the great depression of the 1930s and had adverse effects on their health. This led to an increasing number of conscripts not meeting basic health requirements for basic military services. This is caused alarm at the Department of Defense as it adversely affected the ability to meet the US troop requirements.
As soon as the WWII was finished, the US was already preparing for the next fight with Communism lead by the Soviet Union. It is with this in mind, with planners preparing for the next wave of conscriptions, that the US launched the National School Lunch Act as a means to boost overall health and nutrition for its population.
http://www.educationbug.org/a/the-history-of-the-school-lunch-program.htmlThe History of the School Lunch Program
President Harry S. Truman began the national school lunch program in 1946 as a measure of national security.
He did so after reading a study that revealed many young men had been rejected from the World War II draft due to medical conditions caused by childhood malnutrition. Since that time more than 180 million lunches have been served to American children who attend either a public school or a non-profit private school.
http://education.stateuniversity.com/pages/2395/School-Food-Programs.htmlThe National School Lunch Program
During World War II, many U.S. draftees were found to be malnourished to such an extent that they were turned down for military service. The realization of this low nutritional state of the general population led to the passage of the National School Lunch Act (NSLA) in 1946, which established the National School Lunch Program (NSLP) in elementary and secondary schools. This landmark legislation, although amended many times, continues in force today, with its original objectives still in place. These objectives are "to safeguard the health and well-being of the Nation's children, and to encourage the domestic consumption of nutritious agricultural commodities and other food."