This is all covered inA Good War but I’ll give you the gist of it.
Genn Greymane’s attack on the Forsaken fleet in Stormheim has made the entire Horde justifiably paranoid when they found out that Greymane wasn’t punished for it. This especially worried Sylvanas and Saurfang.
The idea is that even if Anduin is willing to let the peace last if Greymane remains as unchecked as he is now then he will continue to push the Alliance to war.
The biggest problem though is that Orgrimmar is very vulnerable in the wake of the Legion invasion. Saurfang points out that all the Alliance has to do is draw the remainder of the Horde’s navy away and then they can land troops in Durotar and Azshara and then they could just starve Orgrimmar out. Saurfang tells Sylvanas that if that ever comes to pass he will order everyone to charge out of the gates to their deaths because it would be preferable to slowly starving to death. To which Sylvanas replies that it’s her job as Warchief to prevent that.
Saurfang initially thinks any offensive against the Alliance was impossible without terrible consequences but then Sylvanas points out that in the wake of the Legion invasion the Alliance is spread thin and can’t mobilize their forces to respond like they could have before. So the Horde could capture Teldrassil and throw the Alliance into a political crisis that will shatter them allowing the Horde to pick off the Alliance one by one securing favorable peace treaties that will leave the Alliance races pacified for centuries.
Saurfang gives this plan two thumbs up and Sylvanas tasks him with planning the entire campaign. She knows that even though she won over the majority of the Horde populace for her valiant efforts during the Legion invasion, she is still mistrusted by a large portion of the Horde and she feels that they wouldn’t trust her to run an honorable campaign.
Saurfang’s plan is to blitz all the way up to Teldrassil and capture it before Anduin can send reinforcements. Once the tree is under Horde occupation the Alliance would be afraid to besiege it because they might hit the civilians by accident and to get the Night Elves to cooperate he will send Sylvanas to hunt down Malfurion or Tyrande. Whoever is present at the time. Those two are borderline gods. There would be no greater hit to Night Elf morale than see their god-like champion fall to the Horde.
This strategy works for the most part. There was a hiccup when Malfurion raises a wall of wisps to block the Horde advance. Sylvanas managed to get some Horde troops through but the process is extremely painful and straining so she couldn’t do it often. Saurfang gets around this by going around through Felwood to into Darkshore and lighting the forest on fire. It distracts the wisps and lets the Horde army come through.
Everything from that point forward goes smoothly until Malfurion traps Sylvanas and Saurfang intervenes and throws his axe into Malfurion’s back. Which he considered dishonorable. Sylvanas assures him it was fine and allows him to have the honor of taking Malfurions head and she leaves to prepare for take the tree.
Instead of cutting off Malfurion’s head he allows Tyrande to take him to Stormwind. When he reports this to Sylvanas she is furious. The whole point of this invasion was to shatter the Alliance but Malfurion’s survival will be interpreted as a miracle of Elune and the Alliance will be rallied around that miracle. Which will put the Horde in a position like the last war under Garrosh where the Horde and Alliance were trapped in an eternal conflict no one could win.
They continue to move to capture Teldrassil but Sylvanas stops to talk to Captain Summermoon. During this conversation Sylvanas figures out a solution to the problem Saurfang created. She decides this will have to be the final war between the Alliance and Horde. This won’t be a war to last centuries because she will completely annihilate the Alliance. This is how she will save future generations from an endless conflict that Saurfang caused.
So Sylvanas orders the tree to be set aflame. There will be no one seeing this day as a miracle if the Night Elf people are decimated.
Saurfang is furious at first but then Sylvanas points out that everything up until this point was by his design. He failed to inflict the most important blow that his entire plan hinged on. By sparing Malfurion he forced her hand. She needed to find away not to damn the Horde to an eternal conflict because of his failure.
It’s at this point that Saurfang wants to kill her personally. But he doesn’t because he knows she’s right she simply saw the truth before he did. -by [Zezin96](https://www.reddit.com/user/Zezin96/)
exactly