In this case, the Supreme Court ruled that the internment of the Americans of Japanese descent (or at least their removal from the West Coast) was legal. It did so on the grounds of military necessity.
In his majority opinion, Justice Hugo Black relied on the power of the president as commander-in-chief of the military. He argued that this power gives the president the ability to do whatever is reasonable in fighting a war. He argued there was no time for the president and other authorities to determine which Japanese were and were not loyal to the United States. Because it was an emergency, the president’s actions were reasonable and therefore constitutional.