The Rule of Law says that "all persons and authorities within the state, whether public or private, should be bound by and entitled to the benefit of laws publicly made, taking effect (generally) in the future and publicly administered in the courts", according to Tom Bingham in this book. The book traces the history of the concepts contained in the Rule of Law from the Magna Carta through Habeas Corpus (the right to challenge unlawful detention), the abolition of torture, the Petition of Right, the US Constitution, the French Declaration of the Rights of Man, and the law of war, to the Universal Declaration of Human Rights.
The author then goes on to address specifically a number of principles included within the broad concept of the Rule of Law:
* The law must be accessible, clear and predictable
* Legal rights should be resolved by laws and not by discretion
* Laws should apply equally to all unless objective differences justify differentiation
* Public officers must exercise their powers fairly
* Fundamental human rights must be protected
* Effective civil dispute resolution procedures must be provided
* Trials should be fair
* The state must comply with its obligations under international law
Totalitarian regimes down through the ages have been characterised by flagrant disregard for the Rule of Law, for example by having arbitrary laws applied inconsistently at the discretion of the state, granting impunity to a corrupt ruling class, tolerating bribery, imprisoning and torturing people without trial, dispossessing people of property and rights at whim, and murdering prisoners of war who have surrendered. There is not much controversy over whether such practices should be prohibited; the controversy comes when governments enact exceptions to the Rule of Law, arguing special circumstances.
Controversial subjects which the author discusses include terrorism and the Rule of Law, the practice of "extraordinary rendition", discriminatory treatment of non-nationals, detention without trial, hearings in which the accused is not informed of the case against him, torture, surveillance, and the legality of the war in Iraq.
It seems to me that the duty of a state to comply with its obligations under international law is perhaps the most fragile of duties, because the source of those obligations is simply agreements between countries. If one country decides it no longer "agrees", there is very little that can be done to enforce the agreement. Thus the question of when one country has the right to interfere by force in the affairs of another will always be a very difficult one.