Political Analyst, Gwynn Dyer, in a work entitled "War", said "We normally count only the two great wars of our own century as "world wars," but what this phrase means in practice is a war in which all the great powers of the time are involved. By that criterion, there have been six world wars in modern history: the Thirty Years War of 1618-1648, the War of the Spanish Succession 1702-1714, the Seven Years War of 1756-63, the Revolutionary and Napoleonic wars of 1791-1814 and the two World Wars of 1914-1918 and 1939-1945."
There are many who try to include the "Cold War" and the new "War on Terrorism" in the list, making eight. I don’t have a problem with including either. The latter has definitely been declared, though I don’t see any hope of it ever ending as currently defined. It’s certainly possible to win in a war involving particularly named terrorists…but I don’t see it as anything more than waiting for the Messiah to fight against terrorism in general. We’ll stop having terrorists the day all war ceases. But still, this war does involve all the major powers, even if it is inherently unwinnable.
Though it’s a nonsequitor, it looks like we may have 8 planets soon instead of 9. A recent discovery of a mass beyond Pluto, and perhaps slightly larger than Pluto, will force astronomers to either declare there are 10 planets, or redefine Pluto as many have argued for in recent years, as part of an asteroid belt.