For me, the starting point of the Third War is when the prophet, former Guardian of Tirisfal and now Eternal named Medivh began prodding mortals of importance to see their impending doom. Medivh's warnings were taken differently by different people, of course... Thrall, warchief of the remnants of the Horde left behind on Azeroth was more quick to heed the Prophet's warnings than was King Terenas Menethil. But it was the Prophet and his appearance that herals the beginning of the Third War to my mind, even if the war itself doesn't seem as immediately apparent.
Terenas responds to the visitation of the Prophet (who doesn't exactly help matters by not saying who he is, and since I don't know that Terenas ever directly met the man, I can't tell you if he should have recognized him or not - it's fair to say that Jaina Proudmoere, Antonidas' apprentice and a highly placed member of the Kirin Tor, also doesn't recognize the Prophet as Medivh when she directly encounters him) with defiance. The Prophet's message, that Terenas should take his people and abandon Lordaeron, sailing west, didn't sit well with a king who'd managed to sit his throne through the Second War.
In the end, the orc made the right choice: Thrall rallied his people, freed the impetuous Grom Hellscream from captivity at the hands of Lordaeron's soldiers, and stole ships from the humans to sail west. It's not terribly surprising that the orcs chose flight when the humans chose to stay. Lordaeron was hardly home to the orcs of the Horde, after all, it was just the place they found themselves after escaping the internment camps. It's much harder to convince people who've lived somewhere for their entire lives to uproot and head west on the say so of a talking raven that doesn't even bother to identify itself.
While Thrall and Grom were leading the orcs westward, Terenas wasn't idle, nor was Antonidas of the Kirin Tor. Before the Prophet made his appearance, in fact, they were already preparing to investigate a strange series of rumors that were filtering down from the furthest northern reaches of the Eastern Kingdoms. Ironically, the gathering of the orc clans and their seizure of the ships was occupying the majority of Lordaeron's attention at the time (since the orcs were an immediate and known threat, based on their activities during the First and Second Wars) and as a result, there just weren't a lot of resources to investigate. In order to determine the truth, Terenas sent his son Arthas and Antonidas sent his apprentice Jaina Proudmoore to investigate. It most likely seemed like a much better idea at the time than it turned out to be.
We know what happened ultimately to Arthas, of course. He arrived in Brill and then Andorhal to discover that the plague they'd heard rumors of in Lordaeron City was far worse than he ever could have imagined: in Hearthglen they learned that those stricken with it did not merely die but rose from the dead as abominable mockeries of life, soulless corpses that fed on the living. After confronting Kel'Thuzad and slaying him, Arthas and Jaina were visited by the Prophet who again warned against remaining in Lordaeron and demanded that they heed his warning and sail for the west if they wanted to save their people. Like his father, Prince Arthas did not take this message well and refused to abandon his people. Jaina, however, thought that the stranger spoke with a strange authority and considered his message.
The difficulty here is that events unfolded as they did in part due to the meddling of greater powers. The Lich King as it then stood was tinkering with events, sending Kel'Thuzad to found the Cult and hide the plague cauldrons as part of a deliberate plan to create a minion out of the Prince of Lordaeron, but as skilled and powerful as the former Ner'zhul was, it too was being manipulated by the Burning Legion and sought to escape their grasp. At the same time Medivh as the Prophet repeatedly appeared to people he must have known would never listen to him as well as to those he intended to actually aid, imparting his message to Thrall knowing that the orcs would steal ships and leave, knowing that in so doing they would make Kel'Thuzad's job easier. Knowing that in warning Arthas all he was doing was paving the young prince's road to damnation, intending all along that Jaina would take his warning to heart, intending that in Stratholme she would turn away from her childhood love and towards a different destiny. How much of this was the Lich King's meddling, and how much the Prophet? Medivh had contended with Sargeras himself, had fought a war in his very soul with a mind so vast and cosmic in its madness that it dwarfed even that which made the Lich King. Is it too much to believe he intended to sacrifice Lordaeron in order to save Azeroth?
Arthas and Jaina reached Stratholme, finding Uther, and discovering that they'd arrived too late. The plagued grain had already been distributed, the city and its residents already doomed. Their choices were few. They could have attempted to seal the city and wait for the plague to kill and raise every single man, woman and child then strike the horde of undead down as it attempted to spread across the countryside. This course of action would most likely mean horrific casualties, and a chance of their forces also converting into undeath. It's interesting to me that Medivh, who clearly knew the future, didn't feel the need to share it with Arthas or in any way act to spare the people of Stratholme this fate. It's possible he was afraid that doing so would merely force events down even more dire paths and the deaths and undeaths of so many paled in comparison to allowing the Burning Legion free reign across the surface of the world. It's even possible to assume that Medivh viewed the Well of Eternity and the World Tree as far more crucial to protect since they were the prize Archimonde sought as well as the means of his destruction.
The facts remain thus: Medivh knew what was going to happen, and he didn't tell anyone. Vague warnings may have served to protect the ultimate prize, but in the chess game against the lords of the demonic armies, Medivh can be seen as having willingly sacrificed a great many pawns, the people of Lordaeron who died to Arthas, to the plague, and eventually to the Scourge, in order to ultimately take a Legion rook, the Lich King, off the board entirely. (If you've been to Karazhan, you'll note that Medivh is an avid chess player.)
Arthas did not choose to wait out the plague. He did not choose to lay siege to Stratholme and watch the people turn into the undead. In so doing he lost the support of his mentor and his first love, and walked out of a ruined, burned Stratholme burning with a hatred for Mal'Ganis (and not the Lich King himself) that drove him on the path to madness and obsession. And in so doing, he fulfilled both the Lich King's plan for him, and Medivh's plan for the Lich King.
After Stratholme and its destruction (I won't belabor it, if you've done the Culling of Time you've seen the events firsthand) we move into high gear. Arthas seeks his destiny in Northrend, Lordaeron meets its final fate, the orcs make their way across the ocean (meeting surprising new allies at every turn) and all the while, the self-proclaimed Prophet plays with the fate of the world, while the shadow passes ever closer and the Burning Legion edges ever closer to making its way back to Azeroth. Next week, we'll discuss these events, as well as the fall of Quel'Thalas, Grom Hellscream's fall, and the return of the children of the moon.