The Iberian blackout was caused by a lack of reactive power leading to voltages going out of their designed ranges.

The Iberian grid had adequate inertia at the time.

Reasonable grids have forced inverter based generation to manage reactive power for over a decade. But Spain due to lackluster processes relied on thermal plants to provide it. And they didn’t provide the expected amount.

There were some more confounders around oscillations and what not but the central cause came down to voltage control. Which is near trivial to solve in 2026. If you just force everyone to do it.