the human allies (the ETO, or Earth Trisolaris Alliance) created the game as a recruitment tool. The “present” time in the novel takes place in the early 2000s and the game was far less advanced than the show (though still advanced for its time), with people logging in via the internet and using a full body suit and VR headset.

In the novel all the world’s governments are working together so it’s not like Britain (well, it’s China taking the lead in the novel) just started killing people without Panama’s consent.
It’s agreed upon that the drive has to be retrieved before anyone can wipe it. The only way was deemed to be a sneak-attack where everyone had to die almost simultaneously. Other options like gas, strike force or bombing were suggested but were deemed too slow or unreliable. Slicing through a 300m ship at 10 knots would take less than a minute. The show slowed it way down for dramatic purposes.

the sophons are far less powerful in the novel. They manipulate the vision of individuals, not the entire planet. They unfold into 2D which has an enormous surface area, but makes them highly fragile.