Why was Ner’zhul’s first decision to open more portals instead of invading Azeroth ?

So in the lore Ner’zhul gets the idea to open more portals so the orcs could have other planets to invade. He has people hunt down artifacts, alerting the alliance in the process and just flees from them holding them off so he can do his portals, then he blows up the planet.

Ner’zhul realistically could have leveraged his horde against the already weak and rebuilding kingdom of stormwind. He also had Rend’s dark horde nearby who he also could have leveraged into help him. Ner’zhul had a multitude of clans at his disposal as well as powerful lore characters such as kargath, killrogg, grommash, mogor, zuluhed. And teron’gor. The alliance was already fatigued from war and starting to have cracks show like gilneas leaving because they were mad at lordaeron, same with kultiras and alterac had been destroyed. I think he could have secured the southern eastern kingdoms before the alliance mounted an offensive and potentially could have struck a peace due to his forces at his disposal.