When Charles Taylor's NPFL invaded Liberia from Côte d'Ivoire on 24 December 1989 and the Doe regime collapsed into a multi-faction civil war, ECOWAS — under Nigerian initiative at its Banjul summit of 7 August 1990 — created the ECOWAS Cease-fire Monitoring Group (ECOMOG). The first ECOMOG contingent of 3,000 troops landed at the Freeport of Monrovia on 24 August 1990 aboard Nigerian Navy LSTs NNS Ambe and NNS Ofiom, under Lt-Gen. Arnold Quainoo (Ghana) and then Maj-Gen. Joshua Dogonyaro (Nigeria).
Nigerian troop strength peaked at 12,000 — over 80 % of the force — drawn from the 1, 2 and 82 Divisions. Successive Nigerian Force Commanders included Generals Dogonyaro, Rufus Kupolati, Ishaya Bakut, Adetunji Olurin, John Inienger, Victor Malu and Felix Mujakperuo. ECOMOG conducted Operation Octopus (October 1992) against the NPFL siege of Monrovia, the August 1995 Abuja Accord enforcement, and the disarmament that produced the July 1997 elections won by Taylor himself.
Nigerian KIA across the seven-year deployment are officially listed at 500 but unofficially exceed 700 (Adebajo 2002). The fiscal cost — never formally audited — is estimated at US$8–12 billion in then-year dollars (Vogt 1992; Adebajo 2002), funded largely through extra-budgetary CBN drawdowns under the Babachan and Abacha regimes. ECOMOG's record is contested: it stopped Taylor from over-running Monrovia in 1990, but multiple human rights reports (HRW 1993, 1994) documented ECOMOG looting, sexual violence and indiscriminate fire, and the operation arguably prolonged the war by entrenching the warlord-faction model it set out to defeat.
The deployment institutionalised Nigerian sub-regional hegemony, established ECOWAS as a security actor, and produced the doctrine that became the African Union's 2002 Constitutive Act Article 4(h) right of humanitarian intervention. See Liberia relations.