The Economic Community of West African States (ECOWAS) is a regional group founded in 1975 when sixteen West African countries signed the Treaty of Lagos (Mauritania has since withdrawn). Its mission is to promote economic, monetary, and political integration in the sub-region; it has also taken on a role in regional security, with the establishment of the ECOWAS Monitoring Group (ECOMOG).
The ECOWAS Monitoring Group (ECOMOG) is a multi-national armed force tasked by ECOWAS in 1990 to intervene in the Liberian Civil War. It's role has since expanded to include initiatives for regional security and peace-keeping, with interventions to control conflict in Sierra Leone and Guinea-Bissau.
Nigeria provides the largest component (both of armed forces and financial resources); Ghana, Guinea, Sierra Leone and Gambia also contribute smaller units.