God's Crucible - Islam and the Making of Europe, 570-1215 (David Levering Lewis, 2008)

For some reason I found this book incredibly helpful in lining up various threads running through this historical period.  It's probably been helpful to go through books like this, this, this, this, this, etc. - perhaps that gave enough background where, via this one, some pieces finally start to fit together and make some sense.

I don't know that the author had a defining theme here, notwithstanding the title.  Mostly, it's a highly readable, highly useful guide to major currents between 570 and 1215 in Rome, Persia, Arabia, (what became) Europe, with a particular focus on Spain.

Thoughts:

1.  Something like the focus on the Western Front in the two world wars - I tend to think that the Roman Empire primarily was taken down via pressure coming from Western Europe - those fierce Germanic tribes, etc.  This author points out that the real manpower was burned in the East - a long series of struggles between Rome and Persia - leaving both exhausted and weak just when Islam began.

2.  Useful discussion of the beginnings of Islam.  Seems reasonable to conclude that it thrived via conquest in a generally hostile environment and that, as with most religions, its tenets evolved as needed in the circumstances.  Violent conquest led to taxes and wealth to keep the machine going.  The territorial achievements were simply amazing.  I don't know why apologists pull quotes out of Koran and say Islam is purely a "religion of peace" - all religions, not to mention 20th-century communist country constitutions, include those kinds of quotes.  The history for all these institutions is what it is.

3.  Islamic conquerors let folks in the conquered territories practice their religions, somewhat.  But they (unlike Muslims) had to pay lots of taxes.  Which encouraged conversions.  Still, it was a more effective practice than feudalism - which didn't really involve a money economy - the king gave land grants to supportive nobles, but this was a finite supply and tended to set up the noble for future conflict.

4.   Some key dates are set forth in this review.  Reiterates early capital at Damascus, then the move to Baghdad around 762.   

5.  Spain was unique.  Visigoth Arians convert to orthodox Christianity; small part of the population, but controlling - and fired with the zeal of converts.  Mistreat Jewish population.  When the Islamic wave finally gets through the Berbers and is ready to approach the Iberian peninsula - in 711 - Jews are helpful to them. Arabs needed Berbers to take Spain; they made fast progress through a divided country. 

6.  Good discussion of Clovis (511) and Merovingians; the palace mayors that became the Carolingians.  This actually was pretty amazing - no other coherent order elsewhere in what later became Europe.  Defeat of the Arabs/Berbers at Tours in 732 (by Charles Martel, his family not yet royal); not seen as definitive halt of Islamic territorial advance at the time, but divisions among the Arabs/Berbers prevented the next invasions until after Charlemagne et al got their act together and were able to hold the line.

7.  How the western papacy - and Rome itself - hung by a thread.  All the power of old Rome had shifted east to Constantinople.  Rome sacked, population tiny.  Pope powerless other than a mostly-ignored bully pulpit.  Subordinate to Constantinople.  This could easily have turned out very differently than it did.  Carolingians weren't legitimate successors to Merovingians and decided to support the pope in exchange for the pope legitimatizing them.  Turned into a highly significant partnership.  Charlemagne gave the popes the Papal States that were a key dividing factor in Italy for the next thousand years.

8.  Islamic rulers of Spain generally tolerant - but in significant part because they didn't have the numbers to do anything else in the early going when the patterns were set.  Always a tiny minority.

9.  Sophistication in Spain compared to backwardness in Europe.  Charlemagne a truly remarkable figure - both in battle and in imposing Christianity, promoting education etc.  (Even if it barely survived him.)

10.  Charlemagne and Pope - set the pattern for mutual reinforcement of Church and State - forced conversions - orthodoxy - two wealthy power players.  But ambiguity:  which had ultimate authority?  Three-part society - those who fought, those who prayed, those who labored. 

11.   Charlemagne invades Spain to attack Muslims, but has to leave to deal with Saxon uprising.  Rearguard with loot is defeated somewhere near Pyrenees.   Year 775.  Some couple hundred or so years later, this episode becomes The Song of Roland - a hero tale that paints the Muslim as evil, supports wiping them out of Spain (and/or the overall Crusades), etc.  This becomes the template.

12.  Islamic rule in Spain for centuries, varying strength and weakness.  Cultural, architectural, agricultural, economic, educational wonders.  Kingdoms/states in northern Iberian peninsula eventually become powerful, leading to final expulsion in 1492.

13.  After Charlemagne - pretty quick breakup of the "Holy Roman Empire", Viking raids, a mess.

14.  The Islamic state in Spain (al Andalus) - different than the Baghdad branch.