Until about 1500 C.E. history largely revolved around the relationship between two ways of life that people have followed since the birth of agriculture: nomadic herding and settled farming. Environment largely determined how these peoples lived, with wetter climates or river valleys favoring settled farming and drier climates leading to the nomadic way of life. These peoples often co-existed peacefully, exchanging goods and ideas in peaceful trade. But at other times, clashes would frequently occur either because of population pressures forcing the nomads to try to take more land, nomadic jealousy of the richer civilization’s goods, or just mutual hostility between the two ways of life.
Each side had its own advantages in such conflicts. On the one hand, civilized peoples usually outnumbered the nomads since agriculture could support more people than nomadic herding could. Also, their armies generally had better organization, discipline, equipment and technology. On the other hand, the nomads, being more involved with animals, had more meat and protein in their diets, making them bigger and stronger than the farmers.
Probably even more decisive was the nomads' mobility, which let them choose the time and place in which to attack the more settled farmers and cities. Mobility also made it harder for slower civilized armies to catch them. Finally, since nomads often lived on land unsuited to farming, it was not usually worth the civilized armies' time and trouble to try and conquer them, even if they could catch them. This, plus their size, often gave nomads a psychological edge against the farmers, which in any given battle, could be the most decisive element in determining which army would break and run.
Still, as long as a civilization was well governed, its economy healthy, and its armies well trained and disciplined, it was very difficult for a few nomads to prevail. Not until civilization experienced internal troubles such as civil wars, famine, or a breakdown in the government and military organization, could the nomads strike effectively. Typically, they would do this in small-scale isolated attacks, not in one overwhelming wave. Repeatedly raiding the farms, stealing the livestock, and burning the crops, the underlying basis for civilization, over a period of years would trigger a further breakdown in the government, economy, and defense. This, of course, would lead to further raids, more serious breakdowns, and so on. At the same time, the nomads often infiltrated civilization as merchants, settlers, slaves, and mercenaries (professional soldiers). Eventually, the civilization would be so weakened that the nomads could take over. However, this was just the start of a cycle of civilized decline, revival, and expansion that would repeat itself throughout most of recorded history.
After a nomadic takeover, civilization would continue to decline either because the nomads did not care to keep it going, or they cared but just did not know how. What largely determined their attitude toward civilization was the length of contact they had had with it. Generally the longer the contact with civilization, the more it influenced the nomads and made them want to try to continue it. For example, the Saxons who conquered Roman Britain had little prior contact with the Romans and were quite willing to obliterate any signs of Roman civilization they found. On the other hand, such tribes as the Franks and Visigoths who had been exposed to Roman culture for two centuries tried to adopt Roman titles, copy Roman government, live in Roman style villas, wear Roman togas, and even speak Latin.
However, even if the new nomadic masters tried to carry on the old civilized ways, they usually failed because they did not fully understand how the government, record keeping, and technology worked. As a result, the civilization would continue to break down despite their efforts. The damaged economy might not be able to support schools to train civil servants, or the new masters might not even understand the schools' importance. Therefore as civil servants died off, there would be no new civil servants to take their place. Such vital public works as roads and irrigation canals would not be kept up, and the economy would further decline, making it even harder to maintain an efficient government. For whatever reasons, either neglect or the inability to understand how civilization worked, the decline would continue for decades, generations, or even centuries, as was the case with Europe after the fall of Rome.
Despite all this, there were forces working in favor of civilization's recovery. First of all, extended contact with civilization gradually made the nomads more willing to try to preserve it. This at least slowed the rate of decline. Also, the greater material comforts of civilization, such as sleeping on a soft bed or in a warm dry house, might change the nomads' attitudes toward civilized life. Finally, and possibly most important, many nomadic men would take civilized wives. Their sons, although part of the nomadic ruling class, would also be influenced by their civilized mothers to be more accepting of civilized ways. They might also marry civilized women and further dilute the nomadic influence in their children. Eventually, the distinction between the nomads and the civilized people they ruled would virtually disappear, and with it any nomadic hostility toward civilization.
Gradually, the semi-nomadic masters, with their still somewhat restless nomadic spirit, would rebuild civilization to its previous level and expand it beyond that to new frontiers, both culturally and geographically. Of course, the revived civilization would meet new nomadic tribes, and the process would start all over again: new clashes with nomads, their eventual victory in a time of civilized weakness, the further decline of civilization, its revival largely through intermarriage, and its further expansion to new frontiers.
This goes a long way toward explaining much of human history. Of course, each situation had its own particular twists and turns. But the pattern has repeated itself again and again, spreading civilization from such isolated centers as Egypt, Mesopotamia, India, China, Mexico, and Peru. For example, from Mesopotamia and Egypt, civilization would spread to Syria and Palestine, up to Asia Minor, and from there to Greece. The Greeks would bring civilization to Rome and the Western Mediterranean. From there it would spread to northern Europe, and eventually the Americas. If we add other important elements such as colonization and trade, we can view history as the gradual but steady march of civilization across the planet. Taken in that light, one might see history as progress rather than an endless series of wars.