According to Wikipedia, there was somewhere around 15,163,603 to 17,989,982 deaths which can be attributed to World War I, in which this number also included civilian deaths due to military activity, crimes against humanity, and war related famine or diseases. A mere 21 years later was World War II, in which Wikipedia estimated that there was around 60,000,000 to 85,000,000 deaths attributed to the worse war in the history of mankind. It has now been nearly 70 years since the end of World War II and we have yet to have World War III and it doesn't appear all that likely that this will occur in the near future, for this mankind should be eternally grateful.
Since the ending of World War II, there have been plenty of wars in and around the world in which many people have died, been injured, or displaced. Yet, these wars have been getting smaller and smaller and more localized as time has gone on. The last big war in the sense of casualties for the United States was Vietnam and that ended nearly 40 years ago in 1975. Since that time, America has been involved in numerous foreign wars but the footprint of these wars have been relatively small in both scale and time, with the exception of our recent incursions in Iraq and Afghanistan, but despite the length of time we have spent in these two countries, our casualty numbers are significantly lower than either Korea or Vietnam.
Most of World War I and of World War II were fought on European shores, but countries such as Germany and France, that have historically been enemies for centuries, are now both members of NATO, and France is Germany's top exporter of goods. While America and Russia suffered through a "cold war" for over 40 years, there was never an actual war, and through effective diplomacy along with the dissolution of the old Soviet Union, the cold war faded away in 1991.
Perhaps the biggest change over the last 25 years is the ascent of China. China is most definitely the "wild card" in the geopolitics of potential war, as the things that China needs for its continued growth and strength are similar to what we desire as well as other countries. China imports oil, chemicals, machinery, technology, and has a massive population to feed and to care for, to which continued economic progress and expansion is critical for its ongoing success. Because China has a population of over 1.3 billon peoples and an oversupply of males because of their "one child policy", it seems logical that China might get involved in some localized disputes with other countries, such as is has with Tibet, as well as other neighboring nations that they could flex their muscles on.
There isn't a lot of doubt that as China becomes more of an economic powerhouse it will desire that its global influence become greater than it is today. However, this is somewhat mitigated by the fact that it is oil that runs the economic engine of China and therefore it is in China's best interest that they actually work hand-in-hand with United States, NATO, and other nations to assure themselves that the safe transportation and ready availability of oil continues without any undue interruption.
Therefore it can be stated, that war is bad for business, and as long as the access to oil and other important minerals, chemicals, and technology is smooth and continual, World War III is not possible.