Most of the territory in Western Europe that was once part of the Frankish Empire have historically been divided into Duchies and Counties rather than kingdoms. After the division of Charlemagne's empire, Kings of France and Germany arose, but it took centuries to consolidate their power. Germany was never recognized as an sovereign state until 1870.
Regions that were never part of the Frankish Empire, such as Britain (Wessex, Mercia, Wales, etc.), Spain (Leon, Austuria, Castile), Southern Italy (Naples, Sicily) and Eastern Europe (Bohemia, Hungary) are more likely to have independent kings.
Kingdoms of the British Isles
Anglo-Saxon Heptarchy (600-830) — By 600 A.D., when Augustine of Kent embarked on his mission to convert the Saxons of Britain, there were roughly seven Anglo-Saxon kingdoms in the region. These were Mercia, East Anglia, and Northumbria in the North, and Wessex, Essex, Sussex, and Kent in the south. Important Christian kings of the era were Ethelbert of Kent and Offa of Mercia.
Wessex (825) — The Anglo-Saxon kingdoms of Britain not united under a single Christian king until Egbert of Wessex defeated the king of Mercia in 825. A permanent union was not effective until 50 years later under Alfred the Great when a strong king was needed to resist the Danish menace.
England (1066) — When William the Conqueror imposed Norman rule on England after the Battle of Hastings, the government became much more centralized. The Church became wealthy under the Normans, but less independent and the greatest British saints of the Norman opposed the monarchy's interference in Church governance.
Gwyneed/Wales (1070) — The Welsh population was composed mostly of British Celts who fled to the West as the Saxons overran Eastern Britain. Many were already Christianized and others were converted by Celtic monks from Ireland. The Wales was composed of petty kingdoms instead of a central government until it was made a 'March' by the Norman government.
Picts/Scotland (843) — The Christian kingdom of Scotland dates back to the reign of Kenneth MacAlpaine, king of the Picts who united the Picts and Scots in 843 and brought the Stone of Destiny to the Scone Abbey, where Scottish kings were crowned for over 400 years.
Celts/Ireland (450)
Kingdoms of Medieval Spain
Asturias-Galicia (718-924) — First Christian Kingdom established in Spain after the fall of the Visigoth Kingdom. After 200 years, incorporated into Leon.
Leon (910-1230) — Originally split off from the Kingdom of Asturias, but became predominant Christian kingdom in the tenth and eleventh centuries.
Castile (1065- ) — Began as a county on the western border of Asturias-Leon. Became a kingdom in the 13th century. United with crown of Aragon.
Navarre (824- ) — Basque province of Roman Hispania and Visigoth Spain that became independent Kingdom of Pamplona/Navarre after Moslem conquest of Spain.
Eastern Europe - Slavic Kingdoms
Bohemia (870+ ) — Czech kingdoms founded by Western Slaves recently Christianized by Cyril and Methodius.
Moravia (870+ ) — Maravia was a Margraviate (fortified border region) under the Duchy of Bohemia.
Poland (1025+ ) — Slavic kingdom established by Boleslaw I as after St. Adalbert of Prague was sent as missionary to the Slavs.
Pomerania (1121+ ) — Established as a vassal kingdom of Poland. Pomeranians were a coastal Slavic tribe.
Prussia (1525+ ) — Duchy established when the Grandmaster of the Teutonic Knights adopted the Lutheran faith and secularized the order.
Croatia (925+ ) — Croats were a Slavic tribe, Christianized by Rome, that settled in the Balkans in Avar territory. Maintained loyalty to Western, rather than Eastern Orthodox Church after the East-West Schism.
Serbia (780+ ) — Slav tribes migrated to the Balkans in the 8th centuries and converted to Christianity. The Serbs were allied with the Byzantines and became Orthodox Christians after the East-West Schism.
Eastern Europe - Magyars, Balts, Bulgars, Estonians-Finns
Hungary (1000+ ) — Established as a Christian kingdom when St. Stephen, king of the Magyars, was crowned on Christmas day in 1000 A.D.
Lithuania (1236+ ) — Kingdom of Baltic Christians established when Mindaugas was crowned first Catholic king of Lithuania.
Bulgaria (864+ ) — Bulgars were Turkic tribe that migrated into Byzantine territory in the 7th century. Boris I of Bulgaria, established the Bulgarian Orthodox Church and supported the missionaries Cyril and Methodius.
Nordic Kingdoms
Denmark (986 ) — Sven Forkbeard became the first Christian kings crowned in Denmark in 986 A.D.. His son 'Canute the Great' ruled over Denmark, Norway, and England.
Norway (1000+ ) — Harald Fairhair ruled over the first united tribe of pagan Vikings in the 9th century, but Olaf Tryggvason (995-1000) St. Olaf (reigned 1015-28) are recognized as the first Christian kings of the realm.
Sweden (1050+ ) — St. Ansgar established the first Christian mission in Sweden in the 9th century, but it was not governed as a Christian country until the House of Stenkil began to rule in 1050.
The European system of peerage is a complicated one that took root in the Middle Ages as the sovereign nations of Western Christendom came into being. In European history, not all kingships were hereditary. The King of Germany, a title attributed to the ruler of the Eastern division of the Frankish Empire, was elected but tended to remain in the hands of the same house for several generations. The first King of France, Hugh Capet, was elected, but the French Crown became hereditary afterward. In Britain, from the Tudor period onward, the legitimacy of the British crown was increasingly subject to political influence.
Emperors and Kings are Sovereign rulers who do not pay homage to any Feudal overlord. Dukes, Earls, and Counts each rule over a principality, but recognize a king or emperor as their liege-lord. Some Duchies, such as Saxony or Normandy, were larger and more powerful than many kingdoms. Kingdoms were not always stable. They could grow due to conquest or marriage alliances, or they could break up or be divided after the death of a sovereign.
Most of the Christian kingdoms of Europe became hereditary, but in Central Europe, the Holy Roman Emperor was elected from among the existing Dukes and Barons of the region. There was a trend toward hereditary princes over time because they were more stable than elected princes. Some of the benefits of hereditary princes were increased stability, better outlooks for strategic marriages, and fewer wars of succession. Rules of inheritance regarding brothers, sons, and daughters of a sovereign differed by kingdom.