The truth of the matter is that no 2 democracies have EVER gone to war against each other.
A democracy is a nation or government in which EVERYONE, including children, have the ability to vote. As such, there has never been a war between 2 democracies.
Also, as many people have realized, the United States, as along with most of the nations we call democratic, is a republic - we elect officials; we don't vote for bills, as in a pure democracy.
However, the majority of the wars claimed to have been fought between democracies is actually between a Republic and a Monarchy; the War of 1812 and the American Revolutionary War. The Spanish Civil War was technically with a constitutional monarchy; a republic and a republic. In the Indo-Pakistani Wars, Pakistan was a dictatorship - and restricts non-Muslims from voting.
Therefore, its practically impossible for 2 democracies to have ever gone to war with each other.