Sylvanas's diplomacy is so good, she shrugs off Anduin's last desperate peace initiative and launches into another disastrous war for the Horde. Is she so bad at history that she doesn't remember how these have worked out in the past?
When azerite was discovered, she could have cordoned off the area and sent emergency message to Anduin that they needed to talk. She could have ordered guards to prevent mining, but allow Alliance delegates to inspect so there would be transparency. They could have agreed to split it evenly for mutual deterrence, or worked together to make sure no one used it.
She chose none of those, and look where it's leading us: Teldrassil burned, Undercity uninhabitable... And we're just getting started!