Building to Louise's awakening, and the implementation of that power to save Earth from a trigger-happy war, meant adding that One Big Line. Where a summer tentpole might send a hero diving for a red "STOP" button or popping two bullets in the villain's back just in the nick of time, Arrival's big climax is phoned in -- literally.
To sway General Chang (Tzi Ma) from launching a ballistic attack on the alien ships, Louise drifts through space-time for answers. We see her flash forward to meet the general at a fancy gala, where he thanks her for reminding him of his wife's dying words. Then, back in the main timeline, she dials him on his personal cellphone while barricaded in a government base airlock and utters his wife's words back to him. The wisdom resonates enough for him to pull back his forces. The alien ships take off. Their work here is done. It's a bit mind-boggling, but the simple explanation is that, with the past, present, and future at her disposal, Louise is able to retrieve her answers from a timeline where she'd already delivered it.