The study of history is a branch of the humanities.

Literal

Historical-studies [topic-は] humanities [of-の] one-branch [copula-である].

歴史学 is an academic discipline suffix — ~学 forms names of fields: 歴史学 (history as a field), 生物学 (biology), 哲学 (philosophy). 人文科学 ('humanities') contrasts with 自然科学 ('natural sciences'). である is formal copula.