Two flights to Kalimdor
After the culling of the people of Stratholme, a heartsick Jaina Proudmoore had done what no one else would among her peers. Terenas, Antonidas, Uther, even Arthas had failed to heed the warnings of the mysterious Prophet, but Jaina had ultimately decided to follow his urgings and gathered as many people as would follow her. Together, her ragtag fleet sailed west. But even before Jaina, the Prophet had succeeded in motivating the wandering orcs by convincing their warchief Thrall, who rescued his ally Grom Hellscream from the soldiers of Lordaeron and stole a great many of their ships as well.
The orc fleet's trip west was hardly uneventful. While Arthas battled the growing plague in Lordaeron and lost himself in Northrend, the orcs found themselves stranded on an island near the Maelstrom itself. They were soon broken up, with some orcs under Grom Hellscream and the Warsong clan landing on the mainland of Kalimdor before the bulk of their forces. The majority of the orcs were stranded on a chain of remote islands.
Here is where Thrall first demonstrated his having learned the lessons of his predecessor, Orgrim Doomhammer. It was here that Thrall encountered and allied with Sen'jin and his Darkspear Tribe, and it is here that they defeated the forces of the humans and later those of the Sea Witch and cemented that alliance. The Darkspear came to find the orcs under Thrall powerful allies who drove human settlements, murloc raiders and a powerful naga before them, and even though Sen'jin died, his son Vol'jin agreed to bring his trolls under the banner of the Horde.
Likewise, Thrall quickly cemented an alliance with another group upon arriving on the shores of Kalimdor. The tauren under Cairne Bloodhoof made common cause with the orcs against the local centaurs, and upon Thrall's assisting the tauren in leaving the Barrens for the grasslands of Mulgore, Cairne and other tauren warriors decided to aid Thrall and his growing Horde. Cairne had heard of a mysterious oracle in the Stonetalon Mountains and offered to help Thrall find it.
While this was all going on, Grom Hellscream had been fighting with Jaina's forces, who had settled up and down the coast of Kalimdor as well. Both groups, orcs and humans alike, had been noticed by the night elves, and the first contact between the races of the Eastern Kingdoms and the kaldorei were anything but peaceful. Combined with the appearance of Scourge and Legion forces on the continent, the night elves didn't see any of these new races as anything but a threat at first.
This was exacerbated by Grom Hellscream. Upon arriving on the shores of Kalimdor, Grom immediately fell back into his usual "kill all humans" pattern, fighting skirmishes up and down the coast with the forces Jaina had led to Kalimdor. In an attempt to curb Grom's bellicose attitudes, Thrall sent him and his Warsong north to find sources of lumber, since the lands the orcs had already started to settle were fairly devoid of such.
Not welcome here
What ended up happening next was a clash of cultural ideas. To orcs, even orcs who weren't under the influence of the Burning Legion, cutting down a bunch of trees for the wood just isn't that big a deal. Even back on Nagrand, orcs had been a nomadic hunter society with a shamanistic faith that emphasized the elemental spirits. After the coming of the Legion, the orcs had to rebuild a way of life barely remembered from scraps left behind, the vast majority of their shamanistic lore gone, their culture warped forever by the ideals of Horde and Warchief that had been recreated by Gul'dan.
The night elves, however, were not at all a shamanistic people. They revered nature as a whole, the interplay of the natural world, and not the constituent elements that helped make it up. Their animistic, nature-worshiping ways were not ignorant of the idea of nature red in tooth and claw, and the night elves were both capable and willing to unleash feral rage upon anyone they saw as trespassing in their ancestral lands. Such as Ashenvale, the land that Grom Hellscream and his Warsong came to in search of wood for the Horde. Soon, orc and elf were at war, and even more intolerable to Grom, the demigod Cenarius took offense at the orcish clearcutting campaign and stepped in.
Grom was a lot of things ... the first orc to drink demon blood, a fearsome warrior, an unapologetic hater of humans, and above all, he was a partisan for his tribe, then the orcish people as a whole. Seeing warriors of the Warsong cut down at the hands of Cenarius, unable to scratch the wood god, pushed Grom to his limits. Unknown to him, the Burning Legion had since completed scouring Lordaeron and, rather than turn south to destroy the rest of the Eastern Kingdoms, they had crossed the ocean.
Indeed, while Archimonde planned the destruction of the World Tree, his servants Tichondrius and Mannoroth had followed the trail of the orcs and discovered them in conflict with the very same night elves who had blocked the Legion's original invasion 10,000 years before. Mannoroth was angry that the orcs had failed to destroy the humans where the Scourge had seemingly succeeded in Lordaeron, and Tichondrius wanted to destroy Cenarius before he could call the remaining ancients and oppose the Legion again. In the name of killing two birds with one stone, they hit upon the idea of corrupting Grom and his orcs again and spilled some of Mannoroth's blood into a fountain for Grom to find.
We've discussed Grom in length before, so let us just sum it up. Grom was led to the tainted waters by some troll witch doctors who then warned him that they were evil; being Grom, he drank and ordered his Warsong orcs to do likewise. They used the power of these waters to slay Cenarius, only to fall under the control of the Legion again.
As this happened, Thrall and Jaina had each separately reached the oracle in the Stonetalon Mountains, only for him to reveal the truth: that the Burning Legion was at hand, that Hellscream had been corrupted, and that the humans under Jaina's command would have to unite with Thrall's Horde to defeat the Legion and save all of Azeroth. Amazingly, Jaina agreed to help Cairne and Thrall reclaim Grom from his corrupted state by providing Thrall with a Soul Gem to use to capture Grom's spirit. After a vicious battle, the allied forces managed to reclaim and redeem Grom, although many of the Warsong who were too far gone were slain (as were many of Thrall's forces in the fighting). Ashamed of his lapse, Grom told Thrall what Mannoroth had told him, and together, the two orcs sought the Pit Lord out.
While their confrontation with Mannorth was heroic and Grom's death a noble sacrifice that helped free the orcish people from the legacy of their demonic slavery, in terms of the war, it was equally important in removing a major force from the Legion at a critical juncture. However, Tichondrius had taken the opportunity Grom had given him by slaying Cenarius to push deeper into Ashenvale and even start corrupting the forests north of it. The night elves, having failed to stop the orcs and then the demons, were pushed further back.