What jumps out at me are the many distinct grammatical subjects in the modern text compared to the older book:
Published in 2005:Five main clauses, 5 different grammatical subjects.
The German States Remain Separate
German kings after Frederick, including his grandson Frederick II, continued their attempts to revive Charlemagne’s empire and his alliance with the Church. This policy led to wars with Italian cities and to further clashes with the Pope. Conflicts were one reason why the feudal states of Germany did not unify during the Middle Ages. Another reason was that the system of German princes electing the king weakened royal authority. German rulers controlled fewer royal lands to use as a base of power than French and English kings of the same period, who, as you will learn in Chapter 14, were establishing strong central authority.
Published in 1914:Four main clauses, 1 grammatical subject.
In his lifelong attempt to maintain what he thought to be his rights as emperor he met, quite naturally, with the three old difficulties. He had constantly to be fighting his rivals and rebellious vassals in Germany; he had to face the opposition of the popes, who never forgot the claims that Gregory VII had made to control the emperor as well as other rulers. Lastly, in trying to keep hold of northern Italy, which he believed to belong to his empire, he spent a great deal of time with but slight results. (1914)
I'm wondering whether I can improve the 2005 paragraph just by tinkering with the subjects...
OK, here's a rewrite:
Revision with "consistent grammatical subjects":Six main clauses, only 2 different grammatical subjects, with 5 of the six subjects being the same ("German kings").
The German kings after Frederick, including his grandson Frederick II, continued Frederick's efforts to revive Charlemagne's empire and his alliance with the Church, but they did not succeed. Like Frederick, they incited fruitless wars with Italian cities and further clashes with the Pope, and the constant conflict undermined their ability to unify Germany's feudal states under one king. The kings were further weakened by the German political system, which allowed German princes to elect the king, and by their relative lack of royal lands compared to the large territories controlled by French and English kings of the same period, who, as you will learn in Chapter 14, were establishing strong central authority in their own countries. Frederick's successors succeeded neither in reviving the empire nor in unifying their country.
So I guess the lesson is: if you're going to engage in excessive summary, be sure to keep your sentence subjects consistent!