If you look at World History, you will find that dominant political forms have been the Family, the Tribe, the Kingdom, and the Empire, with each level having authority over the levels below it. These all can easily be understood to be varieties of the King-With-Advisors form, different only in their levels of organization. The King consults with his advisors, then makes decisions which must be followed by all of the people and organizations below him. And in most cases, the ultimate authority is believed to have come from the God(s), down to the King, then down to the People. So all of these forms are examples of the Unitary Executive theory.
Rules of conduct are needed, so laws develop over time, usually based on custom, usually with the consent of the people, guided by each person's innate sense of Justice. (The concept of Justice goes all the way back to earliest known Humanity, and all the way around the world to all known people, and has not changed. Justice is accomplished when each person receives what he is entitled to receive, according to the rules he has agreed to, impartially applied. The purpose of the Law is to administer Justice.) As needed, the King can establish new laws or simply organize and codify the existing laws. And as the Unitary Executive, the King has the authority to make whatever laws he chooses, even if his advisors speak against them, and even if the People don't want them.
When everything works well, the King-With-Advisors form of government, within the context of just laws, provides Justice and Happiness to the people, as shown in the Bible under the reign of "Good King Josiah," and elsewhere.
But sometimes the Kings are not so good. "Absolute power corrupts absolutely." Then Injustice and Unhappiness arise, and the people demand something better. This happened in England in the year 1215 when the Barons (essentially Chiefs) under the King felt that the King was abusing his authority, and they forced him to accept a bill of rights known as the Magna Carta. This document guaranteed that "all free men of our Kingdom," and their heirs, would have a certain list of rights "forever," and that nobody could take them away. This was a major break from the Unitary Executive theory, in that the authority of the Unitary Executive (the King) had been severely limited by those beneath him in authority. The Signing of the Magna Carta is still remembered today as a major milestone in the march of Humanity toward Liberty under Law.
An even greater break from the Unitary Executive theory occurred in 1776 when the English colonies in America declared their complete independence from the King and established their own form of self-government under a written Constitution. Under this Constitution (now our Constitution), the people were to govern themselves, with no King, through their elected Representatives to Congress. And a President was to be elected to faithfully execute whatever Laws Congress might enact.
After the American Revolution in 1776, following our lead, much of the World began to move away from the Unitary Executive form of government, to some form of representative Democracy, roughly similar to ours. In country after country, Kings were replaced or severely restricted by elected legislative bodies like our Congress or the English Parliament.
But then, only a few years after World War I, the harshest, starkest modern example of the Unitary Executive theory emerged unexpectedly in Germany. Adolph Hitler outlined his basic Unitary Executive theory in his book MEIN KAMPF, ("My Struggle") written in 1921, prior to his rise to power. He called it the "Leadership Principle," or "Fuehrerprinzip." He completely denied the authority of the People and of their elected Representatives. And he claimed all power for the Leader (der Fuehrer), who is the Unitary Executive. Hitler said . . .
"The folkish State, therefore, has to free the entire leadership - especially the highest, that means the political leadership - from the parliamentary principle of the decision by majority, that means decision by the masses, in order to establish firmly in its place the right of the person."
"There must be no decisions by majority, but only responsible persons, and the word 'council' is once more reduced to its original meaning. At every man's side there stand councillors, indeed, but one man decides."
"The principle which once made the Prussian army the most marvelous instrument of the German people has to be some day in a transformed meaning the principle of the construction of our whole State constitution: authority of every leader towards below and responsibility towards above."
After Hitler came to power in January 30, 1933, he quickly put his Unitary Executive theory (Fuehrerprinzip) into practice. A dictatorship was imposed, and World War II was begun by him a few years later.
For more information about the Unitary Executive theory, please see www.questia.com and http://writ.news.findlaw.com and google.com