By the time you know whether or not another civilization is hostile, it’s probably too late. “Interstellar war” sounds like a long dragged-out set of engagements when it’s more likely that one random day without warning your planet intersects a sizable chunk of tungsten traveling at 0.9c.
Another reason not to confine your civilisation to one planet, or to planets generally.
Much harder to wipe out a civilisation that's dispersed among hundreds or thousands of smaller space colonies. Especially if many of those colonies are hidden in an asteroid belt of millions of rocks.
Whatever scattered remnants that are left of humanity hanging out in the asteroid belt are going to have a hell of a time finding food to eat or oxygen to breathe.
Better to strike first than gamble.