Russia’s crossing into Ukraine last year was illegal, Chairman of the Joint Chiefs of Staff Army Gen. Martin E. Dempsey said during a meeting with Estonian military and civilian leaders in Talinn, the country’s capital. “I believe we have a very close and mutual understanding concerning the threats that aggressive Russia is posing toward the eastern flank of NATO,” Dempsey said. To curb Russian ambitions to move into other neighboring nations, the U.S. has rotated roughly 5,000 troops through Estonia as part of Operation Atlantic Resolve since April 2014, according to the Pentagon.