For the last four years, China has been holding an increasing number of exercises to test their ability to come to the aid of North Korea, in the event of a war with South Korea (and the United States). On in the event of a collapse of the North Korean government. More Chinese ground units have been moved to areas just across the Yalu river from North Korea (and many of the troops set to work guarding the border against the growing number of desperate North Koreans trying to get out.) There have also been exercises with engineers practicing erecting bridges across the rivers that form most of the border between North Korea and China (on the assumption that U.S. or South Korean warplanes would quickly destroy the existing bridges.) The latest development has been the deployment of Su-27 fighters (the most modern in the Chinese Air Force) to the border area. Formerly, most of these Su-27s were stationed near the coast opposite Taiwan.