We prove Serre's conjecture for the case of Galois representations
of Serre's weight $2$ and level $1$. We do this by combining the
potential modularity results of Taylor and lowering the level for
Hilbert modular forms with a Galois descent argument, properties of
universal deformation rings, and the non-existence of $p$-adic
Barsotti-Tate conductor $1$ Galois representations proved in
[Dieulefait, L.: Existence of families of Galois representations and new
cases of the Fontaine-Mazur conjecture. J. Reine Angew. Math. 577 (2004),
147-151].