If one regards the Witenagemot as the legitimate 11th century body to decide who the King of England should be in cases of contested succession, then William the Conqueror was a usurper. But I’ll bet John Locke regarded many of the successors to William the Conqueror as legitimate rulers. How can that be? The key is that anyone who ascended to the throne from a power politics point of view as the successor to a usurper has the opportunity to make their case to the people to become a ruler by the consent of the governed. Back then, this was not always done by submitting to an election, but still involved trying to appeal to the people. For example, Queen Elizabeth I made many efforts to appeal to the people for support and willingness to be governed by her.

The way John Locke frames things in Chapter XIX of his 2d Treatise on Government: Of Civil Government, “Of the Dissolution of Government” is that both invasion and usurpation return people to a state of nature in their obligations. Then they have the right to decide on whether they agree with a particular proposal for a new government with a new ruler:

§. 211. HE that will with any clearness speak of the dissolution of government, ought in the first place to distinguish between the dissolution of the society and the dissolution of the government. That which makes the community, and brings men out of the loose state of nature, into one politic society, is the agreement which every one has with the rest to incorporate, and act as one body, and so be one distinct commonwealth. The usual, and almost only way whereby this union is dissolved, is the inroad of foreign force making a conquest upon them: for in that case, (not being able to maintain and support themselves, as one entire and independent body) the union belonging to that body which consisted therein, must necessarily cease, and so every one return to the state he was in before, with a liberty to shift for himself, and provide for his own safety, as he thinks fit, in some other society. Whenever the society is dissolved, it is certain the government of that society cannot remain. Thus conquerors swords often cut up governments by the roots, and mangle societies to pieces, separating the subdued or scattered multitude from the protection of, and dependence on, that society, which ought to have preserved them from violence. The world is too well instructed in, and too forward to allow of, this way of dissolving of governments, to need any more to be said of it; and there wants not much argument to prove, that where the society is dissolved, the government cannot remain; that being as impossible, as for the frame of an house to subsist when the materials of it are scattered and dissipated by a whirlwind, or jumbled into a confused heap by an earthquake.  

§. 212. Besides this overturning from without, governments are dissolved from within, First, When the legislative is altered. Civil society being a state of peace, amongst those who are of it, from whom the state of war is excluded by the umpirage which they have provided in their legislative, for the ending all differences that may arise amongst any of them, it is in their legislative, that the members of a commonwealth are united, and combined together into one coherent living body. This is the soul that gives form, life, and unity, to the commonwealth: from hence the several members have their mutual influence, sympathy and connexion: and therefore, when the legislative is broken, or dissolved, dissolution and death follows: for the essence and unity of the society consisting in having one will, the legislative, when once established by the majority, has the declaring, and as it were keeping of that will. The constitution of the legislative is the first and fundamental act of society, whereby provision is made for the continuation of their union, under the direction of persons, and bonds of laws, made by persons authorized thereunto, by the consent and appointment of the people, without which no one man, or number of men, amongst them, can have authority of making laws that shall be binding to the rest. When any one or more, shall take upon them to make laws, whom the people have not appointed so to do, they make laws without authority, which the people are not therefore bound to obey; by which means they come again to be out of subjection, and may constitute to themselves a new legislative, as they think best, being in full liberty to resist the force of those, who without authority would impose any thing upon them. Every one is at the disposure of his own will, when those who had, by the delegation of the society, the declaring of the public will, are excluded from it, and others usurp the place, who have no such authority or delegation.

The lack of obligation to obey laws promulgated by rulers who do not have the consent of the people is highly relevant in the situation in Venezuela, as of June 1, 2019: Nicolas Maduro, who has most of the army behind him, does not currently have the consent of the people in the sense of being able to win a free and fair election, which in addition to being an attractive principle in its own right, was the constitutionally established way of choosing a ruler. Many Venezuelans have therefore transferred their loyalty to Juan Guaidó, who has the consent of a large number of Venezuelans to be their ruler, as well as the diplomatic recognition from many other countries as a ruler chosen by a constitutional process, as least for an interim leading up to an election.

Having a dominant army at one’s command does not make one a legitimate ruler. The people must consent to one heading the government, or there is no legitimacy and no obligation to obey new laws.

To avoid anarchy, a good rule is that old laws, duly enacted by duly chosen rulers should still be obeyed while the struggle with invaders and usurpers is carried on. No one should feel they can commit murder unrelated to the struggle for freedom simply because a usurper is on the throne. And many crimes, such as rape, have no legitimate role in any struggle for freedom. It might be inconvenient that old, duly enacted laws may become somewhat outdated if the struggle for freedom continues on a long time before resolution. But even in those cases, leaning hard toward obeying old, duly-enacted laws is an important shield against anarchy.

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