The XXZ Hamiltonian H = Σ[SˣSˣ + SʸSʸ + ΔSᶻSᶻ] is exactly solvable by Bethe ansatz. Δ<−1: ferromagnetic order; −1<Δ<1: gapless XY phase (critical); Δ>1: gapped Néel (Ising) phase. The energy spectrum shown uses exact diagonalization of the two-magnon sector.