The phrase "when, not if" is a collocation, a standard phrase. It is used in a particular context. The context is that people are thinking of something as a possibility. It's a strong statement. There is almost an implied argument. "When, not if" means "you think it might not happen. You are wrong. I am insisting that it is certain to happen. It's not a possibility, it's a certainty." "If we ever get power plants based on nuclear fusion..." "That's 'when, not if.'" "Will Costco raise its membership fees?" "That's a question of 'when, not if.'" "When, not if sea levels rise..."

"When" implies certainty, while "if" implies a lack of certainty. The speaker was certain there was going to be an attack.

Hello, Baboon. My name is John, nice to meet you. The phrase "when, not if" simply means When something is going to happen. This is a statement of surety. Meaning I know something 'is' going to happen, just not when it will.