What should I know before reading War and Peace? Is it important

If you're looking for pure enjoyment, the less knowledge you have, the happier you will be. If you want to know the outcomes historical events beforehand to see how Tolstoy interprets them differently (though keep in mind its historical fiction, but none of the events themselves have been altered, just unique characters added), do some research on the Napoleonic Wars, religion, and serfdom in Russia during the early 1800s. And this is just a personal tidbit here, though Tolstoy attempts to discredit Napoleon several times in W&P, in my opinion it makes him all the more credible, as he is after all, France's greatest finished masterpiece.