The normal practice is that they become "enemy aliens", and subject to internment (ie, POW-type imprisonment). This also applies to civilians that live in areas that fall under military control, for example, Americans living in the Philippines or in Hong Kong, after these areas were captured by Japan.

An example of this is the US WWII internment of the Japanese population on the west coast. This is pretty normal behavior for countries at war. The reason this particular incident is considered shameful is because it applied to US citizens of Japanese descent, as well as to non-citizens.

It would be interesting to know:

• What international agreements relate to the treatment of incidental enemy aliens

• When this practice began

• Specific examples of enemy civilians treated unusually - ie, slaughtered en masse, expelled rather than imprisoned, exchanged for friendly civilians, etc.

This is actually how the (nonfiction) book Seven Years in Tibet starts. The main character is an Austrian mountain climber in India at the outbreak of WWII and is captured by colonial authorities. He escapes twice, the second time making it over the Himalayas and into Tibet.

I'm sure the movie is middling but the book is a fantastic.