Scenario #1: Attack at the worst time.
World War III has ended. Nations all around the world have suffered heavy losses. Everyone everywhere is mourning their dead. At the worst time, the dolphins attack.
They start out by attacking swimmers (as usual), and then they work their way up to large ships. You must be thinking, "How can dolphins take down a large ship?" The answer is quite simple, they use their high pitched, particle-destructing sonar (Similar to the ultrasound used to breakup kidney stones) to attack the weak spots on the ship or they could gain assistance from whales. After the ships have sunk, the dolphins will use the wreckage to assemble vehicles that allow the dolphins to move about on land. These "dolphin vehicles" could be equipped with primitive weapons such as spears, catapults, or "rock guns." Losses could grow to thousands due to the fact that 50% of the population lives around the coasts.