What were the primary assumptions behind the signing of executive order 9066?

The primary assumptions were that the Executive Branch, in the "prosecution of war", has the right to take every precaution and every action in order to prevent espionage and other domestic threats.

This was tied to the assumption that Japanese Americans presented a danger to the United States, and that their ultimate loyalty was to the Japanese Empire.

The primary assumption behind the signing of Executive Order 9066 was that people of Japanese heritage, even if born in the United States, might have retained loyalty to Japan.

Executive Order 9066 was a United States presidential executive order signed during World War II by United States President Franklin D. Roosevelt on February 19, 1942. This order allowed the Secretary of War to prescribe specific areas as military zones, clearing the way for the incarceration of Japanese Americans, German Americans, and Italian Americans in U.S. concentration camps.

Go back and look at what he did and try to fix his mistakes and get on the right track again