For several centuries, Vietnam has had a low level rebellion in its central highlands. That rugged and thinly populated is the home of several tribal groups that resent control by the lowland, and much more numerous, Vietnamese. During the Vietnam war, American Special Forces organized these tribes into combat units to fight North Vietnamese. These Montagnard ("mountaineers") people have recently turned violent again as more lowland Vietnamese move into the central highlands. There have been several riots and acts of sabotage in the last year. To make matters worse, the Free Vietnam movement, organized by a small group of Vietnamese-Americans, has sent armed groups into Vietnam, mainly for the purpose of recruiting additional members and eventually bringing down the communist Vietnam government. The group operates from Thailand and Cambodia. There have been eleven known incidents in the last two years and several Vietnamese Americans are under arrest in Vietnam and Thailand for these activities. The Vietnam government has avoided blaming the U.S. government for all this, although many Vietnam communists believe the U.S. is actually behind it. But the Vietnam government does not want to antagonize the U.S. government and endanger American investment in Vietnam.