1. Correlates of War (COW) Project: This project, maintained by the University of Michigan, has documented over 270 wars since the 1800s.
2. The Encyclopedia of Wars: Edited by Charles Phillips and Alan Axelrod, this reference book lists over 1,763 conflicts, starting with the Battle of Megiddo in 1457 BC and ending with the Russo-Ukrainian War.
3. Peter Kropotkin's "The State: Its Historic Role": In this book, Kropotkin estimated that from 1496 to 1861, Europe experienced nearly 275 years of war and less than 100 years of peace.
4. "War Before Civilization: The Myth of the Peaceful Savage" by Lawrence H. Keeley: This book suggests that there were approximately 10,000 to 15,000 years of warfare during the Late Pleistocene and Holocene periods.
5. "War and Peace: Armed Conflict and International Order, 1618-2003" by Steven Pinker: Pinker estimates that between 14,000 BCE and 2000 CE, there have been approximately 15,000 wars, resulting in around 1 billion deaths.
6. "The Oxford Companion to Military History" edited by Richard Holmes: This reference work lists major military conflicts from ancient times to the present and estimates that there have been over 170 wars since the year 1000.
7. History of Warfare by John Keegan: Keegan notes that between the first recorded battle (c. 3000 BC) and 2000, there have been around 14,500 wars, with an additional 150,000 smaller actions of varying durations.
It's important to note that these estimates can differ depending on the criteria used to define and categorize conflicts, and some might include both major wars involving multiple nations and smaller skirmishes or rebellions.