It’s interesting you should bring that up. I don’t think there is an overwhelming amount of nationalism in War and Peace. I do, after all, use it to meditate on the wisdom of cosmopolitanism. But there is a certain amount of nationalism in the text, sure.
You can see that everything Tolstoy finds virtuous in men is always to be found in a Russian peasant possessed of “Russian” character. Further, Stalin reproduced passages of the novel for public consumption during World War II to drum up patriotic feelings during Hitler’s invasion.