Why did the Trisolarans allow the Judgment Day to be destroyed?

Question

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:

You're bugs!

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?

Answer 1

The sophons are certainly very fast, but they can't be everywhere on Earth at once, especially if they don't know what they're supposed to be looking out for. In this instance, multiple 'battle commands' were set up, each generating multiple plans. One can assume that this also includes decoy meetings and diversionary tactics.

Right now, all the Battle Command Centers around the globe are drafting up operation plans, and Central will select one within ten hours and begin implementation. The purpose of this meeting is to discuss possible plans of operation, and then report one to three of our best suggestions to Central. Time is of the essence, and we must work efficiently.

It seems likely that the Sophons just weren't there when the meeting happened and didn't catch wind of the plan until after it had been enacted against their allies, noting that the current thinking elsewhere (until Da Shi suggests his really quite unexpected plan) is that attacking the ship is a bad idea.

Answer 2

They simply didn't consider it important enough.

I believe @yaroslav-kornachevskyi is correct in his assessment, and feel free to expand into an answer in place of this one. The Trisolarans, due to a combination of overconfidence and ideological reasons, did not consider Evans and the activities of the Earth-Trisolaris Organization an important part of their plan to invade Earth. Indeed, the fact that some humans were actively collaborating with them was barely mentioned during their discussions and the Earth-Trisolaris Organization was not mentioned at all. They did note that a religious devotion to Trisolaran civilization had sprung up on Earth, but their main concern was that something similar might appear among their own people.

The Trisolaran civilization as it existed at the time of the events in the novel was authoritarian and psychologically repressive, however the action taken by the operator of Listening Post 1379 showed that dissatisfaction existed. It's likely that their leadership was surprised and disturbed by the extent of this dissatisfaction. They were concerned that the humanist values of Earth civilization might prove attractive, and "weaken" the mindset of their people or possibly inspire other mutinous actions.

In order to prevent the spread of "humanist" (Trisolarist?) consciousness, they needed to demonstrate that their adversaries were not rational civilized beings with intrinsic moral value.

Hence, they wanted the listening post operator and others like him to witness humans, in their despair, come to see themselves as no better than insects. Their message to Battle Command Centre, "You're bugs!" was an act of psychological warfare intended to achieve this outcome, and the fact that Wang Miao and Ding Yi abandoned themselves to drunkenness (until Shi Qiang's pep talk in the countryside) showed that it was at least partially effective.

Due to their ideological needs the Trisolarans could not afford to take human resistance (or assistance) very seriously. So they didn't think the destruction of the Judgment Day was all that important.

Answer 3

They wanted it (and ETO) to be destroyed because they told Evan about "The Dark Forest" theory (assuming human already knew about it, but turned out didn't). This is the whole plot for Book 2, you should give it a try.