I was out taking care of things during the day. Four updates tomorrow!
Medivh's arrival came as no surprise to Rex. Archimonde's descent was such a momentous event that, had Medivh not come, Rex would have gone looking for him. News from Kalimdor was still somewhat hard for Rex to obtain. Although he had spent considerable money establishing an intelligence network, it had to span two continents, after all. It could not gather as much information as Medivh, who traveled between them in person.
That was to say nothing of Medivh's current role as a liaison. The first collaboration between the Alliance and the Horde would take place through his mediation.
After his arrival, Archimonde proved unwilling to sit idle, and the races of Kalimdor suffered terribly at his hands. Just as in the original storyline, he set his sights on the Night Elves.
There was no helping that. The Night Elves' World Tree was an irresistible lure. Archimonde's power had fallen drastically after he entered Azeroth. To regain it, he had to destroy the World Tree and absorb its power. Only then could he return to his peak.
Archimonde had planned this before invading Azeroth. Once he regained his full power there, he could sweep across the entire world single-handedly. There was no question of that. Thus, the key to resisting the Burning Legion lay in preventing Archimonde from recovering his strength.
In Azeroth, no power from outside was permitted to exceed the level of a demigod. Take Ragnaros the Firelord: in a fit of rage, he had torn the Redridge Mountains in two, creating a roaring volcano between the halves. What had once been a beautiful, fertile land had become a hellscape of fire. That was Ragnaros's destructive power, and it already exceeded anything a demigod could achieve.