This feast of Christ the King is relatively new compared to other feasts in the Catholic Calendar. It was instituted by Pope Pius XI in 1925 in the encyclical Quas primas. To be celebrated every year “on the last Sunday of the month of October…. We further ordain that the dedication of mankind to the Sacred Heart of Jesus … be renewed yearly … on that day.” 
Pope Pius XI (r. 1922-1939) was Pope in the years immediately following the catastrophe known as World War I. The war was a period of misery, suffering, and carnage. When Pope Pius was elected in 1922, he chose the motto “Pax Christi in Regno Christi,” dedicating himself to renew the Christianity of earlier years, and urging the world to seek “the peace of Christ in the kingdom of Christ.”
The Christianity of earlier years was one in which Church and government officials worked together in an effort (more or less successful) to make a society formed by God’s laws. There was a fairly close union between Church and State. The tiara, a perennial symbol of the papacy was first permitted to be worn by Pope Constantine (r. 708 - 715) on the authority of Justinian II's son and co-emperor Tiberios. The early tiara was a plain white cap, heretofore worn only by the Emperor—the three crowns came later.[
In my Master’s thesis I was able to accurately describe the early thirteenth century pontificate of
Innocent III (1198-1216), who clearly functioned as a power broker in the affairs of Europe and beyond. During his reign he influenced the succession of the Holy Roman Empire, excommunicated King John of England, annulled Magna Carta, mediated disputes between France and England, received kingdoms in Spain, Portugal, Bulgaria, Poland, and England as papal fieInnocent III (1198-1216), who clearly functioned as a power broker in the affairs of Europe and beyond. During his reign he influenced the succession of the Holy Roman Empire, fs, and launched a crusade in the East, as well as the Albigensian crusade into France.
But, by the beginning of the next century (the 14th), kings were becoming jealous of the economic and administrative powers exercised by the Pope and the religious orders—jealous enough to challenge them. Philip IV of France went so far as to arrest the Pope and to murder members of the Order of Knights Templar. (There was a lot of money involved!)
Papal power declined even before the rise of Martin Luther’s Protestantism, which assured the fragmentation of Europe along religious and dynastic lines, and spawned what are simplistically called the “Wars of Religion.” I say “simplistically” because alliances sometimes ignored differences in religion in order to challenge the wealthy and powerful houses of kings.
Such wars flourished even after the treaties of Westphalia in 1648, which left Europe with a number of small nations and a number of religions, Catholic and otherwise.
The so-called “Enlightenment” took the “Reformation” a step further. The newly formed generation of “Positivists” declared that nothing was real unless it was empirically determined through the senses. While Luther taught that everyone had the right to their own “private interpretation of scripture,” modernist man was to have his “private interpretation of reality.” In public and in secret, “enlightened” men worked hard to weaken the influence of those who claimed to interpret morality with authority derived either from the Church or from a royal pedigree. Taxes to pay off the war debt did not endear the authorities to the common people.
During just a little over a century before Pius XI, Europe endured two bloody revolutions—the French and the Russian—which struck at the heart of Western civilization—revolutions which forever altered the concept of “Christendom”—revolutions that struck directly at the concept of Jesu Christ as King. Reestablishing the divine Kingship was the primary aim of the Pope who followed World War I.
In the encyclical Quas primas, Pope Pius wrote:
7. It has long been a common custom to give to Christ the metaphorical title of "King," because of the high degree of perfection whereby He excels all creatures. So He is said to reign "in the hearts of men," both by reason of the keenness of His intellect and the extent of His knowledge, and also because He is very truth, and it is from Him that truth must be obediently received by all mankind. He reigns, too, in the wills of men, for in Him the human will was perfectly and entirely obedient to the Holy Will of God, and further by His grace and inspiration He so subjects our free-will as to incite us to the most noble endeavors. He is King of hearts, too, by reason of His "charity which exceedeth all knowledge." And His mercy and kindness which draw all men to Him, for never has it been known, nor will it ever be, that man be loved so much and so universally as Jesus Christ….
17. It would be a grave error… to say that Christ has no authority whatever in civil affairs, since, by virtue of the absolute empire over all creatures committed to Him by the Father, all things are in His power... If We ordain that the whole Catholic world shall revere Christ as King, We shall minister to the need of the present day, and at the same time provide an excellent remedy for the plague which now infects society. We refer to the plague of secularism, (anti-clericalism), its errors and impious activities. This evil spirit, as you are well aware, Venerable Brethren, has not come into being in one day; it has long lurked beneath the surface.
24. The empire of Christ over all nations was rejected. The right which the Church has from Christ Himself, to teach mankind, to make laws, to govern peoples in all that pertains to their eternal salvation, that right was denied. Then gradually the religion of Christ came to be likened to false religions and to be placed ignominiously on the same level with them. 
“The empire of Christ over all nations was rejected,” wrote Pope Pius, but if “the whole Catholic world shall revere Christ as King … We shall provide an excellent remedy for the plague which now infects society.”
In fifteen years as Pope, Pius issued more than thirty encyclicals, many of which might serve as chapters in a catechism teaching us our duties as subjects of Christ the king. He condemned both Nazism and Communism; he considered the diverse topics of economics, religious unity, persecution, marriage, education, the priesthood, the media, prayer, and spirituality—all from the Catholic perspective necessary to acknowledge the Divine Kingship.
In recent years, modernism has vigorously assaulted “the peace of Christ in the kingdom of Christ.” In our age, one might get the false impression that Jesus Christ is not King, but a mere prince among many princes—Moses, Muhammad, Siddhārtha Buddha, Mahatma Gandhi, the Dalai Lama—maybe even Marx, Mao, and Che Guevara! This dabbling in false religions and false ideologies is an assault on the Kingship of Christ—a violation of God’s Holy Commandments.
At the end of this Mass, in the Presence of Lord Jesus in the Blessed Sacrament, we will renew the “dedication of mankind to the Sacred Heart of Jesus,” requested by Pope Pius. It is rather brief, but it will find us giving ourselves over to our true King, calling upon Him to “be King, O Lord, not only of the faithful who have never forsaken Thee, but also of the prodigal children who have abandoned Thee.”
Join us please, and make the words of dedication your own words. The admonitions of Pope Pius XI have gone largely unheeded. Our prayers and our good Catholic example are absolutely essential. Civilization is in danger—Civilization needs to know that it needs Christ to be its King!
 Charles T. Brusca , “Boniface VIII and the Decline of Papal Power,” Florida Atlantic University, Boca Raton, Florida, December 1993, Page 4.
 Quas primas http://www.newadvent.org/library/docs_pi11qp.htm para 17, 24