Molecular dynamics simulation of isobaric heating of an H2O lattice with diamond cubic structure reveals a distinct two-step sequence of hydrogen-bond breaking in a temperature range clearly below the melting point. Partial breakage, signaled by an abrupt increase in local atomic coordination and the coupling of intermolecular torsions with intramolecular angle bending, first occurs and leaves the lattice in a fragmented but still ordered state. This is then followed by full breakage at a temperature about 20-30 K higher, bringing the lattice to an orientationally disordered state characterized by a vibrational spectrum similar to that of water.