USA

Each university has their own set of admission requirements. These requirements are mentioned in the programme brochure. For eg Harvard mentions as follows
Each year the department receives nearly 400 applications to the doctoral programme and offers admission to about 6 percentage of applicants. The typical incoming class size is 16 students. The admissions process is extremely competitive, but if you are serious about pursuing a Ph.D. in history, you are encouraged to apply. Successful applications have shared many of the following characteristics
Personal statement that makes clear why the applicant wants to study history in graduate school, and why the applicant wants to study at Harvard. This statement often illustrates the applicants research interests and notes potential advisors
Three strong letters of reference from people who know the applicants writing sample of remarkable quality that asks historical questions
Fluent or nearly fluent in English
Reading ability in two languages other than English
Strong undergraduate, and, if applicable, graduate record, with excellent marks in history courses
High GRE scores.