Following the reign of a strong monarch, one who had managed to reassert a measure of central authority, the centrifigal forces of the Middle Ages would tend to come back with a vengeance, often undoing all that the monarch had spent his life trying to accomplish.
The Franks provide a stereotypical example:  The forces of disorder had asserted themselves several generations after the death of Clovis; as a result, the Frankish empire which he had built had begun to break down.  On that occasion, the decay had been temporarily halted by the rise of a powerful new family - the Carolingian Dynasty - that for several generations recovered Frankish unity.  After the death of Charlemagne, however, the Carolingian Dynasty went downhill; and the forces of decay and division once again reasserted themselves.  This time, however, there was no new family on the scene capable of reuniting the whole empire.
Future generations would come to regard the period of Charlemagne much as they already regarded the Pax Romana, as a great age of peace, prosperity, and unity to be envied and, if possible, imitated.
Key political problem discouraging unity:  Among the Franks, preservation of a unified monarchy was never an easy task, since it was customary for a king to divide his kingdom among his sons, allowing each to be an independent ruler.  Charlemagne had originally been a co-ruler with his brother, Carloman.  He too fully intended to divide his realm among three legitimate sons; however, the death of the  two older ones prevented him from carrying out his plan.
Louis the Pious:  Charlemagne's only surviving son.  Although he inherited the entire empire, he spent much of his reign trying to work out some acceptable division among his own sons.  In his later years, Louis was constantly fighting whichever one of the sons was dissatisfied with the current plan for division; and when he died in 840, the tired old man was on his way to deal with their latest revolt.
Sons of Louis the Pious at the time of his death:
(1)  Lothair (the eldest)
(2)  Louis the German
(3)  Charles the Bald (the youngest, by a second marriage)
After Louis' death, the sons turned to fighting each other over the inheritance.  Lothair tried to claim the entire empire; as a result, Louis and Charles teamed up to prevent him from getting it.
Fontenay [841]:  the battle in which Lothair fought Louis and Charles in an attempt to make good his claim to the empire.  Since the battle resulted in a draw, it became clear that the sons would have to divide the empire.  Fontenay was also the first battle fought primarily between heavy cavalry - the branch of the military which would continue to dominate warfare through much of the Middle Ages.  Thus, it marked the coming of age of the institution of feudalism, that system of social organization which had grown up among the Franks in the late 8th century and been designed to produce heavy cavalry.
Strassbourg Oath [842]:  an oath to remain loyal to their alliance which Charles swore in the presence of Louis's men and Louis swore in the presence of Charles's men.  The version which Louis recited, sworn before men who were the West Frankish region where Charles had his strength, is often considered to be the first clearcut example of a document written in French.  [In other words, by this time, the speech of this part of the western Roman Empire had diverged far enough from its Latin roots to be considered a new language.]
Treaty of Verdun [843]:  the attempt to divide the empire among the grandsons of Charlemagne.  Lothair received the most desireable part - a strip in the middle running from the North Sea down to the Papal States, known as Lotharingia.  It contained northern Italy which, despite the German invasions, was still the most urbanized region in the west.  Louis the German received the territories to the east, while Charles the Bald got the west.
The Treaty of Verdun is important because it set the stage for the evolution of two of Europe's greatest states:  France and Germany.  The kingdom established for Charles would become the genesis of France while Louis' kingdom would eventually evolve into Germany.
Following the death of Lothair and his son, Lothair II, Louis and Charles began fighting over the territory which lay between their two realms.  This set the stage for another enduring theme of European history - the struggle of France and German to dominate the intervening region.  This struggle lasted right into the middle of the 20th century, the last round ending in 1945.
Treaty of Mersen [870]:  first attempt, through diplomacy, to work out a suitable division of these lands lying between France and Germany.  Like most subsequent attempts, it broke down, leading to renewed conflict over the so-called 'middle kingdom.'
Lorraine:  today a province in eastern France.  The name is an abbreviated form of the word 'Lotharingia.'
Second Invasion:  the invasion of Europe by new groups of invaders which took between roughly 800 and 1050.  Like the Geat Invasion by the Germans 400 years earlier, this latestmovement of peoples once again disturbed the peace of Europe.
Groups involved in the second invasions:
(1)  Moslems
(2)  Hungarians (Magyars)
(3)  Vikings (Norsemen)
Note again:  A major cause of "barbarian" success is the weakness of the society under attack.  Like the Germans before them, these new invaders only made real headway against societies weakened by disunity and internal disorders.  While the Romans were strong during the Pax Romana, they had successfully held the Germans in check.  Only when the Roman Empire was declining did the Germans finally begin to enjoy success.  In a similar manner, when the Frankish Empire was strong under the early Carolingians, it held the invaders at bay.  Only as it became weaker following Charlemagne's death, did it fall prey to the Moslems, Vikings, and Hungarians.
(1)  Moslem Threat:  the initial advance of Islam had posed a serious danger to Europe at the beginning of the eighth century when they conquered Spain and, for about a decade, continued northward into France.  Then, following their defeat at the Battle of Tours in 732, they ceased for a time to be a serious threat.  The Frankish empire, when ruled by strong figures like Charles Martel, Pepin, and Charlemagne, was able to keep its Moslem enemies in check.  In fact, Charlemagne even expanded his empire at their expense by taking a slice of Moslem territory in northeastern Spain known as the Spanish March.
(2)  Vikings:  under Charlemagne, their first raids against the Empire's northern frontier were repelled.
(3)  Hungarians:  these newcomers to the east of the Empire did not face any of the strong Carolingians.  However, Charlemagne had demonstrated his power in this region by virtually wiping out their predecessors, the Avars.
However, just decades after Charlemagne's death, the security once enjoyed by the territories he had ruled evaporated; and the population which had known relatively peaceful conditions was again beset by foreign invasions from the north, the east, and the south.
The new Moslem attack:  by the 8th century, Islam controlled more than half of the Mediterranean coastline and the Moslems had readily taken to the sea.  Even the strong Byzantine navy was hard put to drive them back in the east.  Meanwhile, from bases in North Africa, Spain, Sicily, and Crete, Moslem pirates terrorized Christian shipping.   They were especially effective in the west where none of the Christian powers possessed navies capable of combatting them.  Here, Islam controlled the sea and could attack the coasts at will.  They seized advance bases in France and in Italy from which they conducted raids into the interior.  They carried off not only goods and animals, but also thousands of captives for the slave markets of the Islamic world. A Moslem attack continued to be a very real danger to the people of southern Europe until around the year 1000,  when the danger was finally brought under control by the rising merchant cities of the Mediterranean, places such as Venice, Genoa, and Pisa in northern Italy; Marseilles in southern France; and Barcelona in eastern Spain.
The Hungarian attack:  the Hungarians (or, as they call themselves, Magyars) were another group of nomadic horsemen who swept out of central Asia as the Huns had done centuries earlier.  Their language is not Indo-European like that spoken by most of their European neighbors; instead, it is related to Asian languages, such as Turkish.  (In Europe, only the Finns speak a similar language.)  Ironically, it was Charlemagne who originally made it possible for the Hungarians to enter Europe.  He devastated the Avars; leaving a void in eastern Europe which the newly-arrived Hungarians filled the void which their defeat created.  Late in the ninth century, the Hungarians took advantage of the increasing weakness of the Frankish monarchy; and began to push westward.  For over half a century, their armies of swift-moving horsemen terrorized Germany and Italy; and even made their way westward into France.  It was not until the early 10th century that two new German Emperors, Henry I "the Fowler" and his son, Otto the Great, whose family had replaced the last Carolingians, defeated the Hungarians and drove them out of Germany once and for all.  Afterwards, the Hungarians stopped raiding western Europe, settled down, and, around the year 1000, converted to Christianity.
Battle of the River Lech (955):  the battle at which Otto the Great decisively defeated the Hungarians.
The Viking attack:  The Vikings or Norsemen (literally "men of the North) came from the region of Scandanavia, north of the Baltic Sea, from the countries which are today known as Sweden, Denmark, and Norway.  The languages spoken by inhabitants of this region are classified as Germanic.  During the 9th-11th centuries, the Vikings posed the greatest threat of all to the rest of Europe.
At first, the Vikings mounted summer expeditions which returned to Scandanavia every fall laden with plunder.  By the middle of the ninth century, they had begun to winter in Europe; and eventually, if they returned to Scandanavia at all, it was only after a period of years.
At first, their raids were confined to the coasts, but eventually they used Europe's system of rivers to push inland and plunder ever larger areas.  In the mid-9th century, they penetrated the Mediterranean and added their raids to the already existing problem of Moslem piracy.
Especially hard-hit were the monasteries of Europe which the Vikings soon discovered were a rich source of loot.  In fact, it was in this period that many monasteries became fortresses in order to beat off their Viking attackers.
Danegeld ["Danish money"]:  the money Europeans used in order to buy off the Vikings when they were unable to hold back their raids militarily.
Around the end of the ninth century, the Norse began to settle down in the regions they had once raided.  In fact, they proved to be very successful settlers; and the regions colonized by the Norse ranked among the best governed in all of medieval Europe, for example, the Duchy of Normandy along the French coast which was turned over to the Norsemen under Rollo to get them to stop raiding France.
Rurik:  Viking leader who founded a kingdom centered on the town of Novgorod and Kiev that became the genesis of Russia.
The word Russia may come from Slavic "Rus" meaning "Swede".
Reasons behind the Norse invasion of Europe:
1.  Expanding population
2.  Inadequate agricultural base
3.  Love of adventure and fighting
4.  Desire for trade
5.  Internal rivalries
Note:  many of the pressures which drove Norse expansion were similar to those which drove other societies we have looked at [e. g. the Germans and Arabs] to expand.
Valhalla:  The Norse heaven; entered by the man who died with a sword in his hand.
The area of Norse expansion between the 9th and 11th centuries was as great as the Arab expansion in the 7th and 8th centuries.  However, the Viking influence over the regions they conquered was nowhere near as lasting.
Varangians:  Elite military force composed of Norse hired by the Byzantine emperor to act as his bodyguard.
The Vikings followed three separate routes in their expansion:
(1)  Eastern Route: controlled primarily by the Swedes lay through the Baltic Sea to Finland, Russia, and south to the Byzantine Empire
(2)  Middle Route:  dominated by the Danes, followed the coastline of Europe south to England, France, and eventually the Mediterranean
(3)  Outer Route:  followed by the Norwegians and some of the Danes led to Ireland, Iceland, Greenland, and even America
The most spectacular, though not the most profitable of the Viking voyages were the ones that followed this route, far out into the Atlantic, where they established Viking principalities in Ireland, then sailed on to discovered Iceland, Greenland, and even North America.
Anglo-Saxons:  The main Germanic tribes that conquered Britain in the fifth century were the Angles and Saxons, hence, Anglo-Saxon.  Their hold on Britain began to be threatened by the Norse starting in the 9th century.  From then until the mid-11th century, the Norse played an enormous role on the island.
Alfred the Great (871-901):  Anglo-Saxon king and important scholar who slowed Norse penetration, at the expense of partitioning the island.
Danelaw:  Line in eastern England dividing territory seized by the Vikings from territory retained by the Anglo-Saxons.
Canute:  king who ruled over the great Viking kingdom that included Denmark, England, and Norway.
Eric the Red:  Viking chieftain who settled Greenland; superb publicist who named the island that is anything but green, to attract colonists.
Saga:  Epic stories that made up much of Norse literature.  They told the tales of Viking expansion.
Note:  the Parliament of Iceland, which dates to the year 930, rather than the English parliament (1295), is the oldest representative legislature in the world.
Vinland:  settlement founded by the Vikings around the year 1000 in North America.   The Vikings were driven out of Vinland by Native Americans whom they referred to as Skralings, hence, the first victory in the clash of these two cultures went to the indigenous peoples.  At their next encounter, it would be a different story.
Lief Ericsson "the Lucky":  Son of Eric the Red; Viking leader who discovered North America and established Vinland.
Despite the defeat at Vinland, the Norse appear to have maintained some connection with the New World for several hundred years; until the Little Ice Age that set in around 1300 froze the northern seas to the point that such contact was no longer possible.  At that point, they were forced to pull back, not only from America, but from Greenland as well.
Modern archaeology, at a place called Anse aux Meadowes in Labrador, has established that the Norse presence in America some 500 years before Columbus is a reality.