TL;DR: There is a spectrum of copying from regular story-telling that re-uses ideas and themes, to plagiarism, to copyright infringement. These are not all the same and only the last is illegal.
The concept of plagiarism is not clearly defined. There is a spectrum of idea-borrowing and word-for-word copying that exists and some of it is acceptable, but if you copy too much, or the wrong thing, then you can be accused of plagiarism. Generally, it's not illegal to plagiarize but in some contexts it is forbidden (i.e. in school you're supposed to write your own work and not copy someone else's).
Shakespeare based Romeo and Juliet off of an older story. Countless current movies are re-hashes of pre-existing material. Every story that re-tells an older story, in a sense, plagiarizes that story. To some degree this is fine, acceptable, commendable even. It's how storytelling works.
Legally, the problem of copying is usually restricted to copyright infringement. Laws vary by country but lots of "retelling", not to mention any sizable outright copying of text, is usually prohibited by law. However you are intending to copy historical speeches that are certainly not under copyright anymore. Thus even outright word-for-word copying is allowed by law. Whether your readers will like it depends on what you do with it.
Finally, ideas: ideas are generally not copyrightable. Even though J.K. Rowling made Harry Potter famous, she does not have a monopoly on the idea of a boy going to wizard school. Even if you copied many elements of her story, you're not likely to see problems.
A case in point is the Sword of Truth series by Terry Goodkind, which features a convent of sorceresses who serve the Light and try to struggle against the Keeper of the Dead, yet have been infiltrated by Sisters of the Dark who server the Keeper. This sounds remarkably like Robert Jordan's Wheel of Time's Aes Sedai, who are a convent of sorceresses who serve the Light and vow to maintain order and ready for battle against the Dark One who has power over the dead. They deny the existence of the Black Ajah, a secret organization of Darkfriend Aes Sedai who server the Dark One. Occaisionally, the Aes Sedai must contend with the Aiel, a race of desert warriors who raid amongst themselves and forbid outsiders from living in their until a messiah comes and unites them together to forge an unstoppable army that will be pivotal in the cataclysm to come. This reminds many of the Fremen of Dune, a race of desert warriors who raid the Harkonnen until Paul Atreides comes from the planet Caladan to unite them and forge them into a galaxy-spanning fighting force etc.
Chronologically, Dune came first, then the Wheel of Time, then the Sword of Truth. Are these writers plagiarizing the work that came before? Perhaps. Or are they just borrowing certain set pieces to tell a bigger story? What they do with the story is ultimately what matters and makes them worth reading. (or not: Skip the Sword of Truth, it starts out mediocre and becomes terrible).