“The good fighter will be terrible in his onset, and prompt in his decision.”

A reputation for decisive starts deters; pair it with quick judgment and you control the tempo.
Chapter 5’s energy gets a reputation dimension: the good fighter will be terrible in his onset, and prompt in his decision. Terrible here means dreadful to face — arrivals so committed that meeting them is already losing. Paired with promptness, the verse describes tempo as a deterrent: opponents who know you start hard and decide fast prefer not to make you start at all.
Two countries respond to the same tariff. One issues a statement, then studies the matter, then issues a stronger statement. The other announces and executes retaliation within seventy-two hours — precisely, proportionately, and obviously prepared in advance. Nothing about the retaliation’s size needed to be large; its promptness told every future negotiator that deadlines placed in front of this country are load-bearing.
Prompt decision is not rash decision — the seventy-two-hour country had its response drafted long before, which is why it could be both fast and proportionate. Terrible onset is preparation revealed, not recklessness performed. The misreading swaps the actor’s discipline for the audience’s shock, and shock without preparation just escalates things you cannot finish.