Federation is a device towards union and not unity. While making a union, the hitherto sovereign States still desire to retain their individuality, and for that matter, all issues involved are decided by common agreement and all this is incorporated in the Constitution of the newly established State.
The Constitution clearly defines the jurisdiction of the two sets of government, national and regional, it establishes and any change desired to be made therein can be effected by amending the Constitution in accordance with the prescribed procedure.
It means equality of status between the two sets of government and this is one of the cardinal principles of a federal polity. In a unitary government, it is not so. The central government is supreme and its various political sub-divisions are subordinate to it. The central government can make and unmake them, enlarge or diminish their authority at will, as and when it deems necessary. In a federation, the Constitution is supreme.
All cases of conflict of jurisdiction between the central government and the units of a federation or between one unit and another are decided by an impartial judiciary, called the Supreme or the Federal Court. It is the guardian of the Constitution and it can declare any law, federal or that enacted by any of the units, or an executive order unconstitutional and, consequently, inoperative.
A unitary government does not need such a process, because the powers of the various sub-divisions are not their original powers. They are delegated to them by the central government and what it delegates it can withdraw, diminish or increase. The authority of the central government in this respect is undisputed and unchallengeable.
In a federation there are two sets of laws as there are two sets of government, each government legislating on subjects assigned to its jurisdiction. A citizen of a federal State must obey the laws enacted by the national as well as local governments.
It also envisages double citizenship. In a unitary system of government there is single citizenship and a uniform system of law prevails throughout, as in Britain and France. All this means that the loyalty of a citizen is divided and local jealousies and dissensions plague the federal system. A unitary system of government does not suffer from any of these drawbacks.