Chapter XVII of John Locke’s 2d Treatise on Government: Of Civil Government, “Of Usurpation,” has a very simply point, expressed in the title above. By simple usurpation, John Locke means taking a role in a government one is not entitled to, without otherwise changing the form of the government. Increasing the power of that role beyond what that role is entitled to would be tyranny added to usurpation:

§. 197. AS conquest may be called a foreign usurpation, so usurpation is a kind of domestic conquest, with this difference, that an usurper can never have right on his side, it being no usurpation, but where one is got into the possession of what another has a right to. This, so far as it is usurpation, is a change only of persons, but not of the forms and rules of the government: for if the usurper extend his power beyond what of right belonged to the lawful princes, or governors of the commonwealth, it is tyranny added to usurpation.  

Even simple usurpation is a serious violation of the principle of consent. Indeed, John Locke repeatedly compares a violation of the method a state has for choosing its leaders as being akin to anarchy:

§. 198. In all lawful governments, the designation of the persons, who are to bear rule, is as natural and necessary a part as the form of the government itself, and is that which had its establishment originally from the people; the anarchy being much alike, to have no form of government at all; or to agree, that it shall be monarchical, but to appoint no way to design the person that shall have the power, and be the monarch. Hence all commonwealths, with the form of government established, have rules also of appointing those who are to have any share in the public authority, and settled methods of conveying the right to them: for the anarchy is much alike, to have no form of government at all: or to agree that it shall be monarchical, but to appoint no way to know or design the person that shall have the power, and be the monarch. Whoever gets into the exercise of any part of the power, by other ways than what the laws of the community have prescribed, hath no right to be obeyed, though the form of the commonwealth be still preserved; since he is not the person the laws have appointed, and consequently not the person the people have consented to. Nor can such an usurper, or any deriving from him, ever have a title, till the people are both at liberty to consent, and have actually consented to allow, and confirm in him the power he hath till then usurped.

I find John Locke’s comparison of usurpation to anarchy puzzling. Even in the Roman Empire, where usurpation occurred with stunning frequency, the situation was much better than anarchy. And if usurpation occurred only occasionally, the improvement over anarchy might be quite substantial, assuming those usurpations were not combined with tyranny. (It is possible that the word anarchy did not have quite the same connotations in John Locke’s day as it does now.)

For links to other John Locke posts, see these John Locke aggregator posts: 

• John Locke's State of Nature and State of War

• On the Achilles Heel of John Locke's Second Treatise: Slavery and Land Ownership

• John Locke Against Natural Hierarchy

• John Locke's Argument for Limited Government