Aquinas on Governement

The best government of a society (multitudo) is one that is ruled by one person. This is clear from the end of government which is peace. Peaceful unity among his subjects is the end of a ruler, and one ruler, rather than many rulers, is a more proximate cause of unity.

St. Thomas Aquinas, The Summa Against the Gentiles, Bk. IV, ch. 76.