U.S. Defense Secretary Ash Carter says the NATO alliance is considering establishing a rotational ground force in the Baltic states and possibly Poland as a deterrent to Russian aggression, according to AP. Carter made the comment to reporters flying with him Monday to Stuttgart, Germany, home of the U.S. European Command. Carter said the allies are considering a rotational ground force of four battalions, which would mean about 4,000 troops. That would be in addition to, and separate from, a U.S. armored brigade of about 4,200 troops that the U.S. has already committed to deploying to Eastern Europe next February. Carter said the idea of a separate NATO rotational ground force is likely to be further discussed at a NATO meeting in June.