Answer :
Truman may have fired MacArthur because Truman did not like having been previously under the order of MacArthur.
The source of the document needs to be considered.
When analyzing the source of a document a researcher should evaluate any bias that may be presented in the document as well as point-of-view, audience, and purpose.
In the case of the document written by one of MacArthur's commanders, the researcher needs to present the evidence with caution. Understanding the author has bias toward MacArthur and presenting that information in the writing will create an analysis point for the paper. This document could provide a counterclaim for the researcher to present and refute as well.