Defenders of modern Democracy as the best of all possible governments often promote the "Democratic Peace Theory": the proposition that democratic countries rarely or never go to war with each other and this proves democracy's superiority to other forms of government including traditional monarchies. There are many possible objections to this theory (including examples suggesting that it is simply untrue) made by its critics, who are not necessarily monarchists, but I thought of one in particular I'd like to set forth here.
Even if it is true that wars between democracies are rare, it is even more clearly true that since 1945, for 69 years, there have not been any wars between monarchies either, with the ranks of the world's remaining monarchies including both democratic and non-democratic countries. (I think we can discount the surreal 1947 episode in which India and Pakistan briefly went to war with each other while George VI was still technically King of both, a situation both countries quickly dispensed with.) And it is only since 1945 that Democracy as commonly understood today (which tends to exclude countries like the pre-1914 German and Austro-Hungarian Empires where democratic institutions coexisted with monarchs who retained significant powers) has been the norm in Western Europe. It did not become the norm in Eastern Europe until after 1989, and is still not the norm in much of Asia and Africa.
So before 1945, there simply weren't that many democracies around in the Eastern Hemisphere to potentially go to war with each other, and since 1945, monarchies have not gone to war with each other either, suggesting that we in the West have been enjoying an atypical epoch of relative peace (as, for the most part, did Europeans between 1815 and 1914) which cannot be credited to democracy per se. In fact, most wars since 1918, including World War II, can be blamed on the abolition of monarchies. The major wars that have occurred since 1945 have tended to be either civil wars with outside involvement (Korea, Vietnam), or broad coalitions (which have included both monarchies and republics) against a single dictator such as Saddam Hussein, or whatever the "War on Terror" is. "Traditional" wars of countries A, B, & C against countries X, Y, & Z have been rare since 1945, regardless of forms of government. Therefore it is fallacious to compare the many wars involving monarchies before 1945 with the absence of wars between democracies in the only era where democracies have been plentiful, without mentioning that monarchies during this time have been relatively pacific as well.