Only 62% per wikipedia
The Japanese
Of the approximately 110,000 Japanese Americans who were relocated to internment camps during World War 2, 62% of them were American citizens. Half of those interned were children.
Japanese were interned in WW2 not WW1. German & Austria-Hungarian citizens were interned in WW1. German & Italian citizens were interned in WW2. It is a common international practice to intern the citizens of enemy nations during times of war. The real question was if American citizens of Japanese ancestry (or Japanese citizens with US 'green cards') should be interned by the American government because of the threat of disloyality. The US government believed that the Japanese-American population was more likely to be disloyal than the German-American or Italian-American population. Also these others were much too large to intern.
The US and Canada interned their Japanese citizens in horrible camps illegally. The Supreme Court determined the US Government had broken the constitution when they interned the Japanese people, took their homes and possessions and shut down their businesses.The camps were not suitable or habitable for humans to live in. They were kept in these camps until up to a year after the war was over. An excellent book to read is Farewell to Manzanar to learn what the Japanese went through and how mistreated they were when they were innocent citizens.
1. In Britain most adult male enemy aliens were interned (in internment camps), in many cases for a few months only. British Fascists were also interned or held in prisons. 2. The US had similar policies, but in the case of the Japanese even people born in America by Japanese parents and naturalized Japanese were interned as well as actual Japanese citizens.
Yes
Yes, if they are born in the US, they are US citizens.
Japanese Americans living on the west coast of the US were interned for the duration of the war. There was mass hysteria about their supposed lack of loyalty. Strangely enough Japanese Americans in the Hawaiian Islands were not interred.
No.
Yes, US citizens can travel to Australia.
Yes, US citizens can travel to Japan.
Yes, US citizens can travel to Singapore.