Here you want to make the claim that you’re a good candidate for this particular program.
Avoid “I’ve always loved History” introductions; that’s assumed at this level.
Focus on the periods, themes, and questions that drive your passion for History—for example, “I’m interested in exploring the interplay of class and gender in industrializing European societies” or “I’d like to examine American foreign policy vis a vis Russia in the 1930s.” Explain how your undergraduate training has prepared you to tackle this line of research.
Explain how your interests match both the strengths of the program to which you are applying and the research interests of specific faculty members.
A personal statement should not be a long document: 2-3 full pages tops.