Answer:
Letters and publications written by Nazi leaders before the Second World War.
Explanation:
This answer is correct because this historian is searching for the motives of these perpetrators, letters and publications can tell their thoughts and conversations.
The Industrial Revolution began in England in the late 18th century, and spread during the 19th century to Belgium, Germany, Northern France, the United States, and Japan. Almost all areas of the world felt the effects of the Industrial Revolution because it divided the world into "have" and "have not" countries, with many of the latter being controlled by the former. England's lead in the Industrial Revolution translated into economic prowess and political power that allowed colonization of other lands, eventually building a worldwide British Empire.
The answer to your question is a
