Why didn't the Trisolarans just destroy Earth if they were so far ahead?
Because destroying Earth would defeat the entire point of coming. Trisolaris is not on a mission of revenge—it is on a mission of migration. Their home world orbits three suns whose chaotic gravity makes stable seasons impossible, so their civilization is repeatedly burned, frozen, and rebuilt from nothing. The single most valuable thing in their universe is a planet with a stable orbit and a livable climate, and Earth is exactly that planet.
To the Trisolarans, Earth is not an enemy fortress. It is the promised land they intend to move into. You do not burn down the house you plan to live in. A four-hundred-year expedition only makes sense if the destination survives intact, which is why annihilation was ruled out on day one. The deeper backstory sits in why Trisolaris chose to invade Earth rather than any nearby world.
Couldn't they at least cripple humanity from a distance?
They did try, and the tool was a pair of protons unfolded into planet-scale supercomputers. But what those machines can do has hard limits. The sophons sabotage high-energy physics experiments so that human fundamental science can never gather reliable data, which locks the technology tree in place. They watch nearly every piece of information on Earth, so humanity keeps no secrets. And they run psychological warfare through fake countdowns and the flickering of the night sky.
Notice what is missing from that list: killing. A sophon is a spy and a jammer, not a weapon. It cannot level a city, sink a fleet, or wipe out a population. It can hold humanity still, but it cannot knock humanity down. That is the whole logic of the plan—freeze human progress below the threshold that could threaten Trisolaris, and let the centuries do the rest.
Did they even have a weapon that could destroy a planet at the start?
No, and this simple physical fact gets overlooked. At the opening of the story the fleet crawls along at a small fraction of lightspeed, and the First Trisolaran Fleet needs roughly four hundred years just to arrive. That was the ceiling of their propulsion, not a stylistic choice to take it slow.
The dimensional strikes and light-speed weapons that can erase a planet from across the galaxy belong to far older civilizations and appear much later in the trilogy. Early-crisis Trisolaris did not have them. All they could send across four light-years was information and a fleet that took centuries. Even the droplet, when it finally arrives, targets humanity's military fleet rather than the planet itself, precisely because the world has to stay habitable.
What were the Trisolarans actually afraid of?
They were afraid that humanity would out-develop them. Human science grows exponentially, and four hundred years is long enough for a civilization to transform. Locking the technology tree, watching everything, and breaking morale were all ways to freeze that dangerous variable and bet that Earth could not build a counter before the fleet landed.
The irony is that the bet failed. Humanity never won militarily, but it found an asymmetric answer inside the dark forest theory. Luo Ji threatened to broadcast Trisolaris's coordinates to the entire galaxy, and that threat created a standoff where destroying Earth would expose the hunter's own position to everyone else in the dark. The reason they never simply destroyed Earth, in the end, is not that they could not—it is that under the rules of the dark forest, exposing your enemy exposes you.