Anduin wanted to attack Zandalar to destabilize their relations with the Horde - but this doesn't make much sense. The Zandalari are a very ancient culture - with an extremely proud sense of heritage, and they are very unlikely to surrender to a young human king. What did he expect by killing Rastakhan but to immediately drive the Zandalari closer into the Horde's arms?
- He also sacrificed the lives of Blademaster Telaamon, and battalions of at least several hundred humans, Gilneans, dwarves, void elves and even night elves (whose population is already all but extinct) for his diversion, and Jaina Proudmoore and Gelbin Mekkatorque were nearly killed as well.
- Zandalar had NOT directly attacked Kul Tiras or the Alliance nations. Anduin imprisoned Talanji without attempting to negotiate with them. And then instead of reasoning with Rastakhan - sending him messages, perhaps, via Valeera or some of his spies, begging him to remain neutral, perhaps - he openly attacked his homeland, massacring his people without hesitation, and then killed him.
Tyrande's reasoning was also difficult to understand.
- Darkshore is a fairly narrow and geographically isolated region, with fairly limited natural resources. The kaldorei had previously little interest in Darkshore, compared with Ashenvale and Hyjal. Other than Bashal'Aran - itself a ruined Highborne city until very recently - it had no other major settlements other than Lor'danel. Presumably, the Horde had not attacked Nordrassil (there were still druids there, as some quests showed). So the night elves still had another place to call home. Not to mention Val'sharah, the Eastern Kingdoms, Pandaria with the Tushui Pandaren, and possibly Feralas. But - Tyrande ignored all this. And focused solely on Darkshore of all places. Without any clear explanation. Even if she did want to fight the Horde - why not in Ashenvale, much larger, strategically located, with far greater numbers of defensible settlements, and resource-abundant, closer to Hyjal, the Barrens, and Orgrimmar?
- Most of the night elves in Darkshore were gone, having escaped or been killed. Lor'danel and Auberdine were already destroyed. So Tyrande's intervention could not stop that anyway. If Anduin promised to reclaim these settlements later on, perhaps after Stromgarde is captured - why is that a horrible idea? Would waiting several months, or even a year have made that much difference? Like, it's horrible for the night elves that their moonwells were blighted, for example...but they could always easily make new ones, like that Suramar quests with Val'sharah refugees shows.
- Tyrande refused to coordinate with Shandris, Maiev or even the other Alliance leaders (she was not even present at the Warfront - it was Maiev who reclaimed Darkshore for their people).
- Tyrande's Battle for Darkshore forced Genn Greymane to devote and divert most of his people and resources from the Eastern Kingdoms and other places to fighting there - including his daughter Tess Greymane, further dividing and weakening the Alliance on several fronts.
- Tyrande and the night elves, sending Malfurion, Shandris, Maiev, Jarod, and their other forces, could have helped win the Battle for Stromgarde much earlier on, perhaps even defeat the Forsaken in the Eastern Kingdoms - instead of focusing on a single and not easily defensible region in a continent overwhelmingly dominated by the Horde.
- Tyrande's winning the Battle for Darkshore (which apparently happened after Sylvanas fled Orgrimmar) did almost nothing to really weaken the Horde's presence elsewhere in Kalimdor.
- Tyrande did not NEED to perform the Night Warrior ritual. She and the night elves had survived and won countless wars before that. The Alliance after winning Stromgarde could have helped her anyway - without her putting herself and others at such terrible risk. And she was still very powerful without Elune's most dangerous power. Now she seems estranged from the Alliance (partly), and she is probably going to die. And as she lost against Sylvanas recently, it's not like it's really helping her avenge her people.
- Tyrande eagerly showed up for Anduin's gift of the Kul Tiran vessel from Jaina - but shortly afterwards refused to appear in the Battle before the Gates of Orgrimmar, openly ignoring Anduin and other Alliance leaders when they needed her assistance and support the most, when the war was nearly at its most dangerous stage. She could have helped them significantly, and personally confronted Sylvanas - perhaps even fought her directly, and with Malfurion's help, possibly matching or even defeating her. But again, of course, neither she nor her beloved husband appeared to bother doing so. And thus, Sylvanas escaped - and shattered the Veil between Life and Death, continuing to torture innocent souls, and also putting the universe in mortal danger. But, hey, at least Darkshore, a beginner zone which even most night elf players have little interest in, is night elven land again, just with a permanent moon hanging over everything.
So Anduin and Tyrande both seem to have been horrible strategists - Anduin attacking Zandalar without any real provocation - they had indeed engaged the Horde, but that was only after Anduin captured their princess without explanation. He believed it would permanently destroy their cooperation - but it only caused most of the Zandalari to hate the Alliance and the Kul Tirans, especially Jaina, with everything they had. And then that enabled Sylvanas and Azshara to destroy most of their naval forces - along with most of the Horde's naval forces, and set in stage what happened next with Highkeeper Ra being killed, and N'Zoth breaking free from the Last Prison. So he failed in that.
And Tyrande also failed by almost dividing her people - and the Alliance - in a critical point of a global conflict - she drew away valuable Gilnean resources, she and the night elves (the druids, the priestesses, the Sentinels, the Highborne, and the Wardens), could all have assisted their friends at Stromgarde or in Kul Tiras, helping them with their problems and defeating their enemies at other warfronts - but she completely refused to do so, and opened a new warfront, one which gave the Alliance another headache, and very limited tangible benefits, short-term and otherwise, as well as causing the beginning of an internal rift that might further weaken and splinter the Alliance in the long-term.
Even Tyrande herself refused to remain in Darkshore. She and Malfurion settled in Nordrassil. So it's not like it's become their second home either.
Edit: The first option was a mistake, meant to be, "Yes, probably" (they had a strategy). If someone could change that, I would be really grateful.