So I'm answering this as a political scientist, and in order to give a better answer or to lead you to where you could find an answer I recommend two datasets that are popular with academics at the moment.
Before I share though, the concept of "war" itself needs a definition, and based on that definition you can get entirely different results. Simply having an event named "_____ War" might not make it a war if we impose certain restrictions on what can or cannot count as war. Another note is that even in the absence of a shooting war, peace might not exist. You need only consider the Cold War or the dreadnought arms races of the early 1900s (as two examples) to understand what I mean.
The first is the Correlates of War dataset, which catalogues all interstate wars from 1816-2007. They have a specific, and high, threshold of what counts as a war (1,000 battle deaths, effective resistance, must have lasted more than a year, etc.) Using this dataset you might be able to find a few years of a lack of interstate war after WWI and WII or during the Pax Brittanica. Note though, that their high threshold does not include civil war, or smaller wars like the 6-Day war or the Falkland Islands war. With this dataset, there is maybe 10-20 years of peace (just guessing based on previous experience, don't have my number-crunching computer with me now).
http://www.correlatesofwar.org/datasets.htm
Another dataset is the PRIO/UPPSALA dataset Armed Conflict Dataset. Their dataset runs from 1945 onward and counts both interstate and intrastate (re: civil war, genocide, mass killings etc., though they also code those things separately in other datasets). Their threshold is 25 battle deaths in a year, with combat occurring over the course of a year to make it into the dataset. In this dataset (I'm fairly sure, though I don't have access to my number-crunching computer right now to confirm) has the entirety of 1945-2011 filled with conflict. So according to your original question of nation, states, or groups, there has been no peace in the 50+ years since the end of WWII.
http://www.pcr.uu.se/research/ucdp/datasets/ucdp_prio_armed_conflict_dataset/
Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again.