The original working title of War and Peace was 1805.
Tolstoy began developing the novel in the early 1860s, initially focusing on Russian society and the Napoleonic campaigns beginning in 1805. The first draft was completed in 1863, and the opening portion appeared serially in the periodical Russkiy Vestnik, or The Russian Messenger, in 1865 under the title 1805. More of that version was published the following year.
Tolstoy later became dissatisfied with the draft and substantially rewrote the novel between 1866 and 1869. As the story expanded beyond the campaigns of 1805 to encompass the 1812 French invasion, the occupation of Moscow, and the aftermath of the Napoleonic Wars, the narrower title no longer suited the work. He ultimately published the complete novel as War and Peace in 1869.
The title 1805 is sometimes described as the name of an early version rather than a formal working title, but it is the accepted answer to this trivia question. The Decembrists was another abandoned project connected to Tolstoy’s historical interests, not the original title of this novel.