Adding on to this question: When was the last fully declared war between two democracies, where neither country is Russia?

This edit sidesteps the entire debate about if Russia is currently considered a democracy or not, which I have heard entirely too many times in my college Polisci classes.

Though this question asks about the "last" fully declared war, I want to remind everyone that the 20 year rule remains in effect. Answers that discuss current events will be removed.