Mike Evans, an embittered environmentalist, hopes that the Trisolarans will destroy human life. To that end, he sets up a powerful transmitting and receiving station aboard a converted oil tanker, which he names Judgment Day, and independently establishes contact with them. He also recruits similarly-minded people and prepares to assist the Trisolarans in their invasion by, among other things, disrupting scientific research so that humans will not be able to defend themselves against the invaders. However, when the (united) human governments eventually decide to act against Evans they succeed easily, destroying the Judgment Day and capturing important information including the contents of the messages that have been exchanged.
From this intelligence, it is learned that sophons, autonomous intelligent devices the size of a proton, have been operating on Earth for some years. Among other things, these devices are capable of eavesdropping and two-way FTL communication. This is confirmed when the Trisolarans send a final message to everyone present at a meeting of Battle Command Centre, which they have clearly been monitoring:
It seems the Trisolarans ought to easily have known of the plans to attack Judgment Day, and could have warned them. By not doing so, they lost a significant advantage. Why didn't they warn them?