Linked from
The 16 pages that link to Invariant subspace, each with the reason it gives.
EigenvalueRelated: An eigenvector spans the simplest nonzero invariant subspace.
Jordan normal formRelated: The spans of initial segments of Jordan chains form invariant subspaces.
Schur's lemmaRelated: Kernels and images of intertwining maps are invariant subspaces.
Engel's theoremRelated: The common annihilated vector spans a one-dimensional invariant subspace.