$V(X^m + Y^m - Z^m)$ (projective Fermat curve) isomorphic to projective line iff $m=1, 2$

$V(X^m + Y^m - Z^m)$ (projective Fermat curve) isomorphic to projective
line iff $m=1, 2$

I've convinced myself that the projective Fermat curve $V(X^m + Y^m - Z^m)
\subset \mathbb{P}^2$ is isomorphic to a projective line if and only if $m
=1$ or $m = 2$, but I'm not sure how to prove this fact.
Are there proofs of this fact available online? Can anyone provide a proof?