The terrifying thing about the Scourge was that it grew in strength like a snowball. Facing undead that didn't know pain or fatigue, the living would eventually be killed no matter how hard they tried to resist. And every time the Scourge killed an enemy, they could transform them into undead to join the army.
Therefore, in just a few months, Arthas had gained control of a massive number of Scourge soldiers. After all of Lordaeron fell, he began to plan to march farther into the Eastern Kingdoms so that the Scourge could spread to the entire continent.
Under the pressure of the surging Scourge, the various countries finally took Medivh's warning seriously, especially Jaina. She had already begun to organize the survivors of Kul Tiras and planned to take ships to the continent of Kalimdor in the west.
