Stay at peace as long as you can, build up your economy, improve your research, and at some point you will become unstoppable. The USA works the same way, but it usually gets more time to build up. The USSR has Germany and Japan to worry about, plus a bunch of minor powers.
Germany and Japan start with the most advanced militaries, and they need to strike while the iron is hot.
The USSR needs to stay friends with is neighbors as long as possible, by any means possible. Avoid getting pulled into early wars by the UK, France, or anyone else. You need Germany to send its army West away from you. Strike when you're ready, not when others beg you to.
