Stresses on the fragile Alliance increased following the Second War due to overflowing internment camps and the taxes that were raised to support them. The lack of a common enemy meant there was no reason to put aside the politics and petty squabbles that had defined the Azerothian nations before the orcs invaded from Draenor during the First War.
In addition to this, a plague had developed in the north. Having investigated the matter, Antonidas discovered that Kel'Thuzad, a member of the Kirin Tor, had been practicing necromancy, and suspected that the plague of the north was magical in nature. Headed by the Archmage, the rulers of Dalaran suggested quarantining the affected areas.
Fearful of the backlash it would create, King Terenas failed to listen to their pleas. It was in this climate that a prophet came to the now elderly Archmage. He warned of the plague, telling Antonidas that Azeroth was lost. The prophet advised Antonidas that the only way to save his people was to head west to the continent of Kalimdor.
But Antonidas was now in his seventies. He was wise, but also increasingly cautious and stubborn. Even though his apprentice, Lady Jaina Proudmore, sensed the prophet's great power and suggested heeding his words, Antonidas dismissed the prophet's words as those of a madman.
The archmage decided instead to continue his efforts to investigate the plague in the north from the safety of his city. See: "The Journal of Archmage Antonidas" for his research on the plague. Ultimately, he sent Jaina to investigate the matter. She was to be accompanied by Prince Arthas Menethil.