The Gemora additionally says that there are three requirements for a Milchemes Reshus. The first is the consent of the Sanhedrin (Sanhedrin 29b). The second is the presence of a Melech (Sanhedrin 20a). The third is consultation with the Urim Vetumim, the ornament worn by the Kohen Godol (Sanhedrin 16b).
A number of meforshim significantly limit each of these three requirements from the Gemora. Rambam does not list the requirement of the Urim Vetumim. Rambam does, however, state elsewhere (Sefer Hamitzvos, perek 14) that the Urim Vetumim are needed.