Research publishing houses that publish non-fiction history. Visit their websites and read their submission guidelines.
Send a query letter to editors of publishing houses that publish material similar to your book. Introduce yourself in the letter and describe your proposed project. Because history books are non-fiction, the entire manuscript doesn't need to be written beforehand.
Send a proposal packet to the editor if she is interested. This will include an outline, overview of your story and chapter-by-chapter outlines, as well as information about yourself and your qualifications. Often times having already done a significant amount of research or having substantial credibility in the topic field, is enough to get a commercial publishing house interested.
Hire a vanity press to print copies of your finished book. These companies specializing in pressing up books at the expense of the author. However, they do not offer any editing, marketing or sales of the book.
Sell this book on your own to individual book stores or through trades shows.
Contact a commercial publisher again with proof of sales orders and increased demand for your book. If you have made substantial revenue, they might want to publish it.
Upload your book to one of several online book publishers. These companies have essentially changed the traditional publishing industry and offer such services as: print-on-demand, editorial, book layout, book cover design, ebooks, print services and even in-store retail availability. Many of these publishers, such as Amazon and Lulu, maintain fully online stores where your book can be downloaded.
Create a website, or join a social networking site where you can promote your book. Increased awareness will drive up sales.
Contact a commercial publisher again with proof of sales orders and increased demand for your book. If you have gained a following and/or had significant sales, they might want to publish it.