Answer:
The formation of the Gentlemen's Agreement.
On December 7th 1941, The US entered WWII when Japan attacked Pearl Harbor. On those day about 113.000 people of Japanese ancestry were living in the west coast, in states like California, Oregon and Washington.
President Roosevelt signed executive order allowing The US Army to designate areas "any person may be excluded". However no person from Japanese ancestry was ever convicted of any act of espionage. While 18 Caucasians were tried for spying for Japan.
After the Pearl Harbor attacks an anti-Japanese prejudice took place all over the states, but mostly in states like California, where the local governments established laws against the increasing immigration of Japanese and Asians all together.
The Land Law which prohibited the ownership of agricultural land by "aliens ineligible to citizenship." In 1920, a stronger Alien Land Act prohibited leasing and sharecropping as well. Both laws were based on the presumption that Asians were aliens ineligible for citizenship, created very complex situations for Japanese living in The US.
