I believe the answer is: "Resistance to civil government (Civil Disobedience)"
The main idea of the civil disobedience is to advocate the people to not ever let the government overrule their conscience .
He pointed Mexican–American War as one of the example because he perceived the war as an act of injustice for the people of Mexico.
The best way that would help Markus understand the central ideas as he reads is that, he will be able to idea this by looking at the other elements in a story. The paragraphs in the story he reads will unify into one theme which will lead him to the central idea. One characteristic of a central idea is that, this includes the general or the universal theme of the story.
A theme that Geoffrey Chaucer develops through these two excerpts is that of treachery, since both King Peter of Spain and King Peter of Cyprus were betrayed and their lives ended tragically. King Peter of Spain was betrayed by a man named Bertrand, who had agreed to protect him in exchange for a great compensation, but who eventually handed him over to his half-brother, Henry, who promised him an even greater reward and who assassinated Peter in his tent in 1369, becoming the new king. This is narrated in the excerpt, where Bertrand is compared not with Oliver of Charlemagne, friend of Charlemagne, but with the knight that betrayed him ("No, Oliver of Charlemagne... such a trap!").
Peter I of Cyprus devoting his short yet intense life to fight Islam, and he led the short yet devastating Alexandrian Crusade, but his life ended abruptly. Betrayed by his wife and by some of his closer knights, he was assassinated in his bed also in 1369. This is also referred in the poem ("That conquered Alexandria... on thy bed!").
To sum up, both excerpts revolve around this theme, which they present very similarly: after emphasizing the deeds of the two historical characters, they finalize by regretting their tragic endings.
