Suppose you have a diophantine problem whose solution is connected with the structure of the p-class group of a number field K. Then you have the following options:
- Use ideal arithmetic in the maximal order OK
- Replace OK by a suitable ring of S-integers with trivial p-class group
- Replace K by the Hilbert class field, which (perhaps) has trivial p-class group.
Experience with descent on elliptic curves has shown me that ultimately, the equations you have to solve in methods 1 and 2 are the same; moreover, the approach using ideals is a lot less technical than using factorial domains in S-integers (the class group relations come back in through the larger rank of the group of S-units). I am certain that the route via the Hilbert class field is even more technical: again, the unit group in the class field will produce more difficulties than a trivial class group will eliminate.
Edit. As an example illustrating my point in a very simple example, let me solve the diophantine equation $x^2 + 5y^2 = z^2$ in several different ways. I will always assume that $\gcd(x,y) = 1$.
1. Elementary Number Theory
The basic idea is factoring: from $5y^2 = (z+x)(z-x)$. Since $d = \gcd(z-x,z+x) = \gcd(2z,z+x) \mid 2$ we have $d = 1$ or $d = 2$; moreover we clearly have $z-x > 0$.
This gives $z+x = da^2$, $z-x = 5db^2$ or $z+x = 5da^2$, $z-x = db^2$. Solving for $x$ and $z$ yields
$$ x = \pm \frac d2 (a^2 - 5b^2), \quad y = dab. $$
2. Parametrization
Set $X = \frac xz$ and $Y = \frac yz$; then $X^2 + 5Y^2 = 1$. Take the line $Y = t(X+1)$ through the obvious point $(-1,0)$; the second point of intersection is given by
$$ X = \frac{1-5t^2}{1+5t^2}, \quad Y = \frac{2t}{1+5t^2}. $$
Dehomogenizing using $t = \frac ba$ and $X = \frac xz$ etc. gives
the projective parametrization
$$ (x:y:z) = (a^2-5b^2:2ab:a^2+5b^2). $$
If $ab$ is odd, all coordinates are even, and we find
$$ x = \frac12(a^2 - 5b^2), \quad y = ab; $$
if $a$ or $b$ is even we get
$$ x = a^2 - 5b^2, \quad y = 2ab $$
as above.
3. Algebraic Number Theory
Consider the factorization
$$ (x + y\sqrt{-5}\,)(x - y\sqrt{-5}\,) = z^2 $$
in the ring of integers of the number field $K = {\mathbb Q}(\sqrt{-5}\,)$.
The class number of $K$ is $2$, and the ideal class is generated by
the prime ideal ${\mathfrak p} = (2,1+\sqrt{-5}\,)$.
The ideal $(x + y\sqrt{-5}, x - y\sqrt{-5}\,)$ is either $(1)$ or
${\mathfrak p}$; thus
$$ (x + y\sqrt{-5}\,) = {\mathfrak a}^2, \quad (x - y\sqrt{-5}\,) =
{\mathfrak b}^2 $$
in the first and
$$ (x + y\sqrt{-5}\,) = {\mathfrak p}{\mathfrak a}^2, \quad
(x - y\sqrt{-5}\,) = {\mathfrak p}{\mathfrak b}^2 $$
in the second case.
The second case is impossible since the left hand side as well as
${\mathfrak a}^2$ are principal, but ${\mathfrak p}$ is not. We
could have seen this immediately since $x$ and $y$ cannot both be odd.
In the first case, assume first that ${\mathfrak a} = (a + b\sqrt{-5}\,)$
is principal. Since the only units in ${\mathcal O}_K$ are $\pm 1$,
this gives $x + y \sqrt{-5} = \pm(a+b\sqrt{-5}\,)^2$ and hence
$$ x = \pm (a^2 - 5b^2), \quad y = \pm 2ab. $$
If ${\mathfrak a}$ is not principal, then
${\mathfrak p}{\mathfrak a} = (a+b\sqrt{-5}\,)$ is,
and from $({\mathfrak p}{\mathfrak a})^2 = 2(x+y\sqrt{-5}\,)$ we
similarly get
$$ x = \pm \frac12(a^2 - 5b^2), \quad y = \pm ab. $$
4. S-Integers
The ring $R = {\mathbb Z}[\sqrt{-5}\,]$ is not a UFD, but $S = R[\frac12]$ is;
in fact, $S$ is even norm-Euclidean for the usual norm in $S$
(the norm is the same as in $R$ except that powers of $2$ are dropped).
It is also easily seen that $S^\times = \langle -1, 2 \rangle$. From
$$ (x + y\sqrt{-5}\,)(x - y\sqrt{-5}\,) = z^2 $$
and the fact that the factors on the left hand side are
coprime we deduce that $x + y\sqrt{-5} = \varepsilon \alpha^2$ for some unit
$\varepsilon \in S^\times$ and some $\alpha \in S$. Subsuming squares into
$\alpha$ we may assume that $\varepsilon \in \{\pm 1, \pm 2\}$. Setting
$\alpha = \frac{a + b\sqrt{-5}}{2^t}$, where we may assume that $a$
and $b$ are not both even, we get
$$ x + y \sqrt{-5} = \varepsilon \frac{a^2 - 5b^2 + 2ab\sqrt{-5}}{2^{2t}}. $$
It is easily seen that we must have $t = 0$ and $\varepsilon = \pm 1$ or
$t = 1$ and $\varepsilon = \pm 2$; a simple calculation then yields the
same formulas as above.
5. Hilbert Class Fields
The Hilbert class field of $K$ is given by $L = K(i)$. It is not
too difficult to show that $L$ has class number $1$ (actually it is
norm-Euclidean), and that its unit group is generated by $i = \sqrt{-1}$
and $\omega = \frac{1+\sqrt{5}}2$ (we only need to know that these
units and their product are not squares). From
$$ (x + y\sqrt{-5}\,)(x - y\sqrt{-5}\,) = z^2 $$
and the fact
that the factors on the left hand side are coprime in ${\mathcal O}_K$
we deduce that $x + y \sqrt{-5} = \varepsilon \alpha^2$. Subsuming
squares into $\alpha^2$ we may assume that
$\varepsilon \in \{1, i, \omega, i\omega \}$. Applying the nontrivial
automorphism of $L/K$ to $x + y \sqrt{-5} = \varepsilon \alpha^2$ we find
$\varepsilon \alpha^2 = \varepsilon' {\alpha'}^2$. Since the ideal
${\mathfrak a} = (\alpha)$ is fixed and since $L/K$ is unramified,
the ideal ${\mathfrak a}$ must be an ideal in ${\mathcal O}_K$.
Thus either ${\mathfrak a} = (a+b\sqrt{-5}\,)$ is principal in $K$,
or ${\mathfrak p} {\mathfrak a} = (a+b\sqrt{-5}\,)$ is; in the second
case we observe
that ${\mathfrak p} = (1+i)$ becomes principal in ${\mathcal O}_L$.
Thus either
$$ x + y \sqrt{-5} = (a+b\sqrt{-5}\,)^2 \quad \text{or} \quad
x + y \sqrt{-5} = i \Big(\frac{a+b\sqrt{-5}}{1+i}\,\Big)^2, $$
giving us the same formulas as above.
Avoiding ideal arithmetic in $K$ and only using the fact that
${\mathcal O}_L$ is a UFD seems to complicate the proof even more.
Edit 2 For good measure . . .
6. Hilbert 90
Consider, as above, the equation $X^2 + 5Y^2 = 1$.
It shows that the element $X + Y \sqrt{-5}$ has norm $1$;
by Hilbert 90, we must have
$$ X + Y \sqrt{-5} = \frac{a+b\sqrt{-5}}{a-b\sqrt{-5}}
= \frac{a^2 - 5b^2 + 2ab\sqrt{-5}}{a^2 + 5b^2}. $$
Dehomogenizing via $X = \frac xz$ and $Y = \frac yz$ yields the same
projective parametrization as above, and we end up with the
familiar formulas.
7. Binary Quadratic Forms
The equation $x^2 + 5y^2 = z^2$ tells us that the form $Q_0(X,Y) = X^2 + 5Y^2$
with fundamental discriminant $\Delta = -20$ represents a square;
this implies that $Q_0$ lies in the principal genus (which is trivial
since $Q_0$ is the principal form), and that the representations of
$z^2$ by $Q_0$ come from composing representations of $z$ by forms
$Q_1$ with $Q_1^2 \sim Q_0$ with themselves.
There are only two forms with discriminant $\Delta$ whose square is
equivalent to $Q_0$: the principal form $Q_0$ itself and the form
$Q_1(X,Y) = 2X^2 + 2XY + 3Y^2$. Thus either
$$ z = Q_0(a,b) = a^2 + 5b^2 \quad \text{or} \quad
z = Q_1(a,b) = 2a^2 + 2ab + 3b^2. $$
The formulas for Gauss composition of forms immediately provide us with
expressions for $x$ and $y$ in terms of $a$ and $b$, but they can also
be checked easily by hand. In the first case, we get
$$ x^2 + 5y^2 = (a^2 + 5b^2)^2 = (a^2 - 5b^2)^2 + 5(2ab)^2, $$
and in the second case we can reduce the equations to this one
by observing that $2Q_1(a,b) = A^2 + 5b^2$ with $A = 2a+b$, which gives
$$ x^2 + 5y^2 = \frac14\Big(A^2 + 5b^2\Big)^2
= \Big(\frac{A^2 - 5b^2}2\Big)^2 + 5(Ab)^2. $$
