What happens when an advanced civilization breaks down and leaves feudalism in its wake?

One case is when the global temperature changes and new areas are favored while other areas are left behind.  The other case is when an advanced civilization becomes an empire and the erosion caused by high levels of military spending steadily reduces the civilization into a lower state of existence.

CLASSIC FEUDALISM

Classic feudalism or medievalism refers to the down period in European development between the end of the Roman Empire and before the Renaissance of the Modern Era.  With the overrunning of the well organized society by barbarians, the breakdown was replaced with rampant warlord militarism leading to many fiefdoms dominated by castles with motes.  The lord would give protection to those working the land in his or her castle at night and from time to time as these workers were called serfs.  The lord serf relationship features high income inequality.  The lack of intellectual development as martial skills are instead rewarded leads to a lack of what we in the developed world call progress in the sciences.  The church steps into the void in scientific progress and becomes a dominant institution.

ROLE REVERSAL

Just as one great religion rose at the peak of the Roman Empire, Christianity, another great religion, Islam, rose at the peak of the Byzantine Empire.  As the Western European world was in decline, the Arab world was in ascendancy with the invention of algebra, the zero, and the spreading of Islam across the Southern Mediterranean and into Eastern Europe and Asia.  The reaction to all of that was the Crusades. 

Then the tide turns with the Renaissance and the Industrial Revolution, as Christianity once more became ascendant. The reaction to all of that were the rejection of colonialism and the rise of Islamic fundamentalism. The recent attacks on the president for mentioning the Crusades in the same breathe as the current religious extremists just goes to show how determined to be wrong the right usually is.  The Middle Age world warming that favored more Northerly Europe over the more Southerly Middle East lead to the Crusades, which lasted about as long as the farming in Greenland, both ending as the world cooled.  The first Crusade when it reached Asia Minor, now called Turkey, descended on a town there where every man woman and child were killed.  It turned out they had unwittingly chosen a Christian community.  So the Crusades of yesteryear did not lack in barbarity in comparison to today's religious militants.

MODERN EMPIRE DECLINE

Today's America seems ascendant in every way on the world stage, but the rot from within is well advanced.  While China and India graduate ten times the engineers the US does, we send our best talent into financial schemes on Wall Street.  As the rich get richer and the middle class fails to obtain pay raises at all, we devolve more and more into a lord serf relationship society.  Indeed, the military seems the only way out for many in poor white rural communities.  When the technology manufacturing sector is in decline and the retail and service sectors are ascendant, that is the surest sign of a declining empire.  When 4% of the world's population houses 25% of the prisoners, crime is at epidemic rates in America.  When America has the highest homicide, prisoner, mental illness, teen birthrate, and obesity rates in the developed world, surely it is because of high income inequality and militarism, as shown in my tables in the Empire Economics presentation listed second immediately below:

More on the climate change scenario:

https://www.academia.edu/4485586/CIVILIZATION_Changes_with_Climate

More on the empire decay scenario:

https://www.academia.edu/4044446/EMPIRE_ECONOMICS_Peer_Review_AWARD