Since the question originally did not specify what action by Roosevelt (or which Roosevelt) we are talking about, I have guessed that it refers to the internment of the people of Japanese descent on the West Coast at the beginning of World War II.
If we need to argue that this action was ethical, we can say that, in war, the overriding goal is to win. Almost all other goals must be subordinated to this goal. This is arguably ethical because, in the example of WWII, an American victory would have been much better for everyone in America (and perhaps everyone in the world) than an Axis victory. If some people had to have their rights trampled in order to secure victory, that is simply the price that had to be paid.
We can argue that this was ethical because it did not mistreat the people of Japanese descent as badly as it might have. They were put in camps, but they were not killed or even treated very poorly (outside of the fact that imprisonment is inherently unpleasant).
So, we could argue that the internment was perfectly proper and ethical. The US government arguably acted with restraint and did so for a worthy cause. If we have to defend the internment, this is the way to go about doing so.