Haig may have known everything about the previous generation of warfare, that doesn't necessarily make him intelligent. It makes him knowledgeable.
Now, he was also likely intelligent enough to apply his knowledge of then conventional warfare to various then conventional situations. And this likely made him excellent at his position. In so much that he was appointed a leader of his nation's army during WWI.
But he lacked the intelligence to achieve anything novel. He lacked the speed of thought to see where the current situations couldn't apply to his breadth of knowledge. He lacked the ability to deduce where his knowledge could be adapted to this new situation.
That's why "smart" people seem "stupid". Because they're likely knowledgeable, not intelligent. Every single person I've considered intelligent, it wasn't because they knew a lot of facts. It's because they're always a couple thoughts ahead of most people. They're able to see what others are getting to, sometimes before the others even know they're about to get there.
Intelligence is a multiplier. It's not a thing on its own. That's why most decent intelligence tests test abstract reasoning.
We also make the mistake of confusing success for intelligence. That if someone is successful, they must be intelligent. That's not the case. Success is multi-faceted and there is no one thing that will make you successful.