Research what companies may be interested in your manuscript. War books target a particular market, and many publishers specialize in that market. Categorize your book and search for a publisher that accepts war manuscripts. This means deciding if it is fiction or non-fiction. It may have other components like drama or intrigue, or it may be specific to a real war. Those labels will help you to identify a publisher. Using a writing industry resource, such as Writer's Market, narrow down the publishers to query. In this industry-standard resource, all publishers are listed in categories defined by the type of work they publish. Their listing includes information about what they specifically want to receive from you, who to send it to and what their acceptance policies include as well as other company information.
Prepare your work for shipping. You will find some publishers will accept whole manuscripts, others will want only a chapter selection. Prepare a cover letter to each publishing company you intend to query. In the cover letter you will want to include your contact information and your manuscript's information, including title and word count. You also want to include a self-addressed stamped envelope. The publishing company will reply to you with this envelope.
Mail out your manuscript queries and wait. You may hear back from someone quite quickly if your book is well written and in an area that is easy to sell, or it may take some time for a response. You do not need to follow up with the publishing company.
When you find a publisher who would like to publish your book, you sign a book deal. This may include edits and rewrites and will also include the terms of payment and possibly additional books on the subject. A publication date will be set for your book to go to market.