(Simple?) applications of Class Field Theory?

You might like Class Field Theory and the First Case of Fermat's Last Theorem by Lenstra and Stevenhagen. The first case of FLT is that we cannot have $$x^p+y^p+z^p=0 \ \mathrm{and} \ xyz \not\equiv 0 \mod p.$$

Lenstra and Stevenhagen use computations with $p$-power reciprocity laws in $\mathbb{Q}(\zeta_p)$ to show that, if the first case of FLT were solvable, than $p$ would have many special properties such as: $$2^{p-1} \equiv 1 \mod p^2$$ $$3^{p-1} \equiv 1 \mod p^2$$ $$2p+1, 4p+1, 8p+1, 10p+1 \ \mbox{are all composite}$$

Using these conditions and ones like them, one can rule out all primes under $10^{18}$. These conditions were already known by other methods; the contribution of this article is to provide very short and uniform proofs from Class Field Theory.

This isn't an easy article -- following all of the details involves understanding Class Field Theory very well. But I think it is as well written as possible, given the subject matter.

In my very first research paper, I needed extensions of $\mathbb{Q}$ with dihedral Galois group and with lots of ramified primes of a particular kind. Even though these are non-abelian extensions, their existence can be shown using class field theory.

I used this to say something about growth of Selmer groups of elliptic curves, which are Diophantine equations in two variables of degree 3. Not sure if that qualifies for you as applications to Diophantine equations.

If you have studied global class field theory then there are many uses. Firstly the Cebotarev density theorem appears as a generalisation of Dirichlet's theorem...and it is a nice little exercise to see how Dirichlet's theorem falls out if you explicitly use the cyclotomic extension $\mathbb{Q}(\zeta_n)/\mathbb{Q}$.

Also the Artin reciprocity law explains all previously known reciprocity laws and provides a more general setting for them...it gives a reciprocity law for every single abelian extension! For example, quadratic reciprocity follows from it. Read Cox to find out how.

Cox also is a master of the subject of primes of the form $x^2 + ny^2$. Suffice to say it is easy to see that this is really the same question as determining which primes split into nice ideals in the order $\mathbb{Z}[\sqrt{-n}]$ of $\mathbb{Q}(\sqrt{-n})$. Depending on what $n$ is you can find major uses for the Hilbert class fields and the ring class fields in getting congruence conditions for such a prime to have the above form! Which path you use depends on whether $\mathbb{Z}[\sqrt{-n}]$ is the full ring of integers.