A large group of Russian troops violated 'military operations 101' and paid the price in a deadly Ukrainian HIMARS strike, experts say

• Ukraine said it launched a deadly HIMARS attack on five Russian units gathered on a beach.
• Russia seriously misjudged its soldiers' proximity to Ukrainian HIMARS, two experts told Insider.
• "Lesson for the Russians: your adversary is clever," one said. "Don't cut your range calculations too closely."

The Russian units targeted in a Ukrainian HIMARS attack this week flouted a key wartime rule, experts told Insider: Don't gather in big, open spaces.