Correlates of War

Research project

The Correlates of War Project (COW) was founded in 1963 by J. David Singer, a political scientist at the University of Michigan. The original and continuing goal of the project has been the systematic accumulation of scientific knowledge about war. Joined by historian Melvin Small, the project began its work by assembling a more accurate data set on the incidence and extent of inter-state and extra-systemic war in the post-Napoleonic period. To do this scientifically Singer and Small found they needed to operationally resolve a number of difficult issues such as what is a “state” and what precisely is a “war.” Building upon the work of other pioneers such as Pitirim Sorokin, Lewis Frye Richardson, and Quincy Wright, Singer and Small published The Wages of War in 1972, a work that established a standard definition of war that has guided the research of hundreds of scholars since its publication. In March 2001 the project was transferred to Penn State, which has archived all available original material from the Correlates of War project, and is extending and enlarging the data collection efforts it began.


The COW project provides and maintains several datasets, listed below. Most of the datasets are hosted by Penn State, but some of them are hosted externally. Only the COW Wars datasets are examined in detail here.

  1. COW Wars Data (Non-State, Intra-State and Inter-State War Data (v4.1)
    Dataset covering military conflicts between or among non-state entities (non-state war), between states (inter-state war), and within states (intra-state war). This data set records such events over the 1816-2007 period.
  2. State System Membership (v2008.1)
    This data set records the fluctuating composition of the state system since 1816. It also identifies countries corresponding to the standard Correlates of War country codes.
  3. Extra- State War (v3.0)
    This data set records extra-state wars over the 1816-1997 period.
  4. Militarized Interstate Disputes (v3.1)
    This data set records all instances of when one state threatened, displayed, or used force against another. Version 3.1 covers the 1816-2001 period.
  5. Militarized Interstate Dispute Locations (v1.1)
    This data set records the geographic locations of MIDs in latitude/longitude coordinates. Version 1.1 covers the 1816-2001 period.
  6. National Material Capabilities (v4.0)
    Six indicators — military expenditure, military personnel, energy consumption, iron and steel production, urban population, and total population — are included in this data set. It serves as the basis for the most widely used indicator of national capability, CINC (Composite Indicator of National Capability) and covers the period 1816-2007.
  7. Formal Alliances (v3.03)
    This data set records all formal alliances among states between 1816 and 2000, including mutual defense pacts, non-aggression treaties, and ententes.
  8. Territorial Change (v4.01)
    This data set records all peaceful and violent changes of territory from 1816-2000.
  9. Direct Contiguity (v3.1)
    The Direct Contiguity data set registers the land and sea borders of all states since the Congress of Vienna, and covers 1816-2006.
  10. Colonial/Dependency Contiguity (v3.0)
    The Colonial/Dependency Contiguity data set registers contiguity relationships between the colonies/dependencies of states (by land and by sea up to 400 miles) from 1816-2002.
  11. Intergovernmental Organizations (v2.3)
    This data set tracks the status and membership of intergovernmental organizations (IGOs) from 1815-2005.
  12. Diplomatic Exchange (v2006.1)
    This data set tracks diplomatic representation at the level of chargé d'affaires, minister, and ambassador between states from 1817-2005.
  13. Bilateral Trade (v2.01)
    This data set tracks total national trade and bilateral trade flows between states from 1870-2006.

In addition to the datasets listed below, the website provides links to several related and compatible datasets. These datasets either employ Correlates of War country codes, derive from, or build upon, core Correlates of War datasets (e.g. alliance, militarized dispute, war, state membership data).

The COW War Data


Correlates of War


Comma-separated-variables (CSV)




100+ historical and present countries

Last reviewed


Data types and sources

Expert-coded variables and information based on qualitative historical sources. For the interstate war data set, a paper providing an annotated bibliography for many of the wars is available for download.

Data download

Correlates of War


The variables in each of the inter-, extra- and intra-state war datasets can roughly be separated into three main categories: time span, geographical scale (location) and a set of incident-specific variables. The variable names and labels are listed on the website for each of the datasets.

Geographical coverage

The datasets covers more than 100 unique historical and present countries.

Time coverage and updates

All three data sets identifies wars and their participants between 1816 and 2007. An updated version of the data (v4.1) was released in March 2011.


A short list of variable names and labels are provided online for each of the datasets. However, there are no codebooks or detailed descriptions of variable constructions or coding rules, but the site states that questions or concerns regarding the data coding rules may be forwarded by email. Also, the site refers to "The Wages of War, 1816-1965: A Statistical Handbook" (Singer and Small, 1972) and "Resort to Arms: International and Civil Wars, 1816-1980" (Small and Singer, 1982) for additional information concerning the procedures followed to generate the data.

Access conditions and cost

Available free of charge.

Access procedures

Predefined table.

Data formats

Comma-separated-variables (CSV)

Comparability and data quality

The work of Singer and Small in the Correlates of War project, especially the operationalization of difficult issues such as what is a “state” and what precisely is a “war”, established a standard definition of war that has guided the research of scholars of conflict since its publication. While it is amongst the most commonly used datasets on conflicts, its operationalizations and definitions of war has also been criticised. Collier and Hoffler (2001) points out that the thresholds involved in the original COW definition of civil war/intra-state war is based on the use of violence and not on the aims of the protagonists or on the outcome of the conflict, which in turn may lead to the exclusion of a substantial number of internal wars. However, the shifting nature of international conflict has prompted a rethinking of the Correlates of War Project’s classification of wars (Sarkees and Schafer, 2000). Thus the definitions of war has been subject to change and adjustments during updates and expansions of the different datasets, which has improved the quality of data provided by COW.


Collier, Paul and Anke Hoeffler. 2001. "Data Issues in the Study of Conflict". Paper prepared for the Conference on "Data Collection on Armed Conflict", Uppsala 8-9 June 2001.

Sarkees, Meredith and Phil Schafer. 2001. "The Correlates of War Data on War: an Update to 1997". Conflict Management and Peace Science, Vol. 18, No. 1, 2000, 123-144.

Singer, J. David, and Melvin Small. 1972. The Wages of War, 1816-1965: A Statistical Handbook. New York: John Wiley.

Small, Melvin, and J. David Singer. 1982. Resort to Arms: International and Civil Wars, 1816-1980. Beverly Hills, Calif.: Sage Publications.