Let $G$ be a group. Let a principal $G$-bundle $G\to E\to B$. Then we have a fiber sequence $G\to E\to B\to BG$.
Let $k$ be a field. Suppose $H^*(BG;k)$ and $H^*(E,k)$ are known. How to get $H^*(B;k)$?
The action of $\pi_1(BG)$ may not act trivially on $H^*(E;k)$. Hence in Serre spectral sequence, local coefficients need to be used. However, local coefficients cohomology is unknown because we are not clear about the action of $\pi_1(BG)$ on $H^*(E,k)$. What should I do?