Why Is Counterterrorism Cooperation So Difficult?

Cooperation in CT is much more feasible at the tactical level (when it is indeed coordination) than at the operational or strategic levels when even defining who is a terrorist is much more difficult when national politics and nationalism are injected. But if we stop trying to look at the whole and would instead focus on the shared areas that are agreed on - would cooperation actually become viable? Could other areas be reached after such a start? The only one that benefits in the end is IS - that is able to keep its enemies divided and to beat them in detail in one or more dimensions across the globe.