This is a great question, Michelle, and a very specific one.
Since you are asking about becoming a specific type of historian, there's no question that you will need to pursue a career in higher education. First, complete a bachelor's degree history. Like any undergraduate degree, you'll be expected to take foundations history courses, courses that given you adequate "coverage" (e.g., courses on the history of different regions and time periods), and courses that seek to increase your knowledge of historical research methods.
If you want to go on to graduate school, you'll need a high GPA (aim for 3.85 or higher), reference letters from professors, one or more high-quality writing samples in your field and for most schools, either the GRE or GMAT with particularly high scores on the verbal and writing sections. Cut off rates for GPAs and GRE or GMAT scores vary from school to school but if you want to gain a spot in a top-ranked graduate program in history, the higher your GPA and test scores, the better.
A critical step in your pathway to becoming a warfare historian will be choosing where you attend gradaute school and with whom you choose to work. While a bachelor's degree in history from any high-quality institution is fine, at the graduate level, the program and supervisor matter a great deal. In your case, you'll want to pursue a master's degree and PhD under the supervision of a professor who is already recognized for their work in warfare history. In the U.S., there are specific history programs known for their work in this field (e.g., UNC's Department of History) and in these departments, you will benefit from finding a cluster of professors who share your interest. Of course, where you pursue your graduate studies and under whose supervision will matter a great deal on your further specialization. If your primary interest is in British military history, you'll likely want to work with a historian who specializes in this area. Of course, they may be more likely to reside in the U.K. than in the U.S. so you will want to look at programs outside the U.S. as well. As a historian, you'll also need to think about access to archives. Again, if you're pursuing research on British warfare history, your primary research materials will likely be in the U.K. so you will benefit from being at a university close to these materials.
When going on the academic job market, bear in mind that jobs in your field will be limited - perhaps, just a few relevant postings a year - so being able to teach in your specialization and in a more general field may help. Also, you should have an appropriate number of publications (ideally, a book or book under contract with a reputable university press) and several refereed scholarly articles on topics in your area of specialization.
For more on teaching in higher education, see ToBecomeATeacher.org's article, "How to Become a College or University Professor."