  Twisted bilayers of transition metal dichalcogenide semiconductors have
enabled the discovery of superconductivity, ferromagnetism, correlated
insulators and a series of new topological phases of matter. However, the
connection between these electronic phases and the underlying band structure
singularities in these materials has remained largely unexplored. Here,
combining the magnetic circular dichroism and electronic compressibility
measurements, we investigate the influence of a van Hove singularity on the
correlated phases in bilayer WSe2 with twist angle between 2-3 degrees. We
demonstrate stabilizing the Stoner ferromagnetism below moir\'e lattice filling
one and Chern insulators at filling one by tuning the van Hove singularity
cross the Fermi level using the electric and magnetic fields. The experimental
observations are supported by the continuum model band structure calculations.
Our results highlight the prospect of engineering the electronic phases by
tunable van Hove singularities.
