Government is to be distinguished from the State, and both are to be distinguished from the nation.
The State is a perfect society ordered towards the earthly happiness of its members. By 'perfect society' is meant a society that possesses all the means necessary to accomplish its end. Thus the Church possesses all the means necessary to achieve its purpose, the heavenly happiness of its members, including a sacerdotal hierarchy (sacerdotium) and sacraments. The State, similarly, has its regnal, or civil, hierarchy (regnum), which ensures the efficient and economic organization and coordination of its members. The State differs, however, from the Church in this respect, namely that, if it is to have more than the merely potential existence of a Platonic idea with no concrete instances, it must be manifested and embodied in some nation, whereas the Church is more concrete and more real than the various national or local 'churches' in which it is instantiated for this reason, namely that the local churches exist only inasmuch as they participate in the one Christ (cf. Colossians 1:18).
The State, then, must be embodied by a nation, but what is a nation? I propose that a nation is a collection of human persons united by consanguinuity, a shared language and shared traditions, a shared land, or, more likely, some combination of the three. Since nations differ in their racial qualities, the riches of their civilization, and the resources of their homeland and are always wanting something which which can only be had by intercourse, commerce, and conflict with other nations, they are never perfect societies in themselves, but always more or less imperfect manifestations of the State, which will only be perfectly realized when the new Jerusalem comes down out of heaven (cf. Apocalypse 21:2)—the new Jerusalem which will have no temple (21:22) because in it, the 'square will be circled,' Church and State will at last be one and the same divine Society ruled by Christ the King of kings, Who is also the High Priest and sacrificial Victim of the New and Eternal Covenant.
The civil hierarchy of the State, concretely embodied in some nation, is a government. (Strictly speaking, the rule of a household by the family patriarch, of a religious community by an abbot or abbess, of an ecclesiastic juridiction by a bishop, and even of the entire world by divine Providence are also governments, but I will simply refer to civil governments as governments since this thread does not concern any other kind.) Now hierarchy in general consists in the asymmetric relation of authority and obedience between a superior who commands and a subordinate who obeys, but when we speak of government, we invariably mean only the commanding element of the civil hierarchy. Therefore the purpose of government is to promote the earthly happiness of a particular nation by issuing commands to its members.
Since a command is one but its object and occasion may be many, government must, in the interest of efficiency and economy, be able to make laws which apply to not just one member of the nation at one time, but all members at all times. Since, moreover, it is not always clear how these laws are to be applied in a given situation, it must also be able to interpret the laws. It is thus seen how the three powers of government, executive, legislative, and judicial, are not independent of each other, but themselves hierarchically ordered: the judicial power is ordered to the legislative, and both to the executive, which alone is government par excellence. In view of these considerations, the most reasonable mode of government, in my opinion, is monarchy, in which the supreme executive, legislative, and judicial authorities are one and the same person, who is truly the national embodiment of the State.
Acting in the interest of the nation is clearly of paramount importance for the government (dissolving the nation by promoting mass immigration, sexual immorality, or subversive ideologies is objectively treason), but should be distinguished from delineating between citizens and non-citizens. It is not necessarily in the interest of the nation that every one of its members should be part of the commanding element of the civil hierarchy, either directly, by filling an office in the government, or indirectly, by having a vote as to who does fill the offices.