Peacekeeping
There was a planned deployment of UN peacekeepers meant to ease these concerns and help with the transition to peace, but things began to unravel. International contributors began to shy away from further peacekeeping initiatives; such as an expensive and open-ended mission in a strategically unimportant country. As a result, the UN's intervention force was slow to come to fruition and then came to a halt completely when Sankoh argued the size of the contingent of 740 UN peacekeepers was too large.
The UN refused to engage without total consent from both parties, thus preventing the deployment of a peacekeeping force. This consent-based approach, Shearer argues, illustrates the limits the UN can play in the volatile and fragile state of affairs that exist during and after civil wars. "In Sierra Leone, it meant that an important component needed to shore up the peace-building process was absent. It also meant that Sankoh was dictating terms." This consent-based approach effectively allowed the leadership of a brutal rebel group to hinder the potential for peace.
The situation was exacerbated by the fact that the newly elected President of Sierra Leone terminated the Executive Outcomes contract undermining his hard power advantage. Things were further inflamed when disaffected officers of the army overthrew the government in 1997. The war quickly renewed. A small UN force of monitors was deployed to observe the security situation. UNOMSIL, as it was called, was deployed between July 1998 and October 1999, but was forced to withdraw from the country when the RUF took the country's capitol.
UNAMSIL was eventually formed and deployed in 1999, authorised under a Chapter VII mandate, it was meant to enforce the Lome agreements. However, violence would continue. From the outset the RUF was beyond uncooperative and once the ECOMOG contingent withdrew, the RUF attacked UN forces, eventually taking hundreds hostage. This led to an unexpected backlash from the international community that the RUF did not anticipate. Its leadership had expected the international community to cut and run, as it had done in Somalia and earlier when UNOMSIL fled Freetown. Instead, with British support, an aggressive campaign was waged against the RUF. UNAMSIL's mandate was expanded and its manpower enlarged. By late 2000 and early 2001 the RUF's military strength had been severely depleted. Thus the Abuja agreements were signed and UNAMSIL fulfilled its mandate in December 2005. While Sierra Leone is at peace today and the UN's mission can be deemed a success, the way in which the situation developed illustrates Shearer's point: that a consent-based approach focused on negotiation that encompasses all belligerents' interest may not necessarily lead to success. As we see, fighting continued despite the presence of UNOMSIL. Indeed, even after UNOMSIL was replaced by a more robust force under a Chapter VII mandate in the form of UNAMSIL the violence continued. When the British intervened militarily and substantially degraded the RUF's capability to sustain the conflict, as Executive Outcomes had done years prior, the RUF finally come to the negotiating table and allowed for the establishment of peace.
Some authors question the idea of international interventions at all. In a 2005 working paper for the Center for Global Development, Jeremy Weinstein of Stanford University provides a theory of "autonomous recovery", in which states can achieve sustainable peace without international intervention. Using case studies of Uganda, Eritrea, and Somalia, Weinstein demonstrates how states can develop effective institutions out of warfare. This method has cost and benefits that must be weighed against the potential outcome of international intervention. External intervention can stop mass atrocities, but also stop institutional change. Autonomous recovery elevates the strongest leader, but also rewards the strongest fighters who may be less inclined to share power. Furthermore, intervention depends on external influence while autonomous recovery is based on internal factors. The conclusions of his argument could suggest intervention is not ideal policy, but Weinstein argues the international community's "responsibility to protect" doctrine has moral importance for intervention and the conditions for "autonomous recovery" are very rare. Weinstein argues the fundamental challenge is how to incentivise good governance and assistance to rebel groups without disrupting the connection of citizens to rulers in terms of revenue collection that enables accountability.

Mission creep: Although acknowledging a number of practical and moral reasons for peacekeeping operations, James Fearon and David Laitin assert that they have a tendency under some circumstances to become tangled with state-building efforts.  In weak states facing successful guerrilla campaigns, peacekeepers face pressures to build state institutional and administrative capacity in order to achieve lasting peace.  These pressures can lead to mission creep beyond the original purview of the peacekeeping operation; without engaging in state-building, the peacekeepers risk allowing the peacekept country to revert to violence following their exit.  Thus, Fearon and Laitin advocate for the greater integration of state-building in peacekeeping efforts through a new framework of "neotrusteeship", which would see foreign powers exercising a great deal of control over a weak state's domestic affairs in order to ensure the prevention of future violence.

Lack of engagement with the populace: A growing critique of peacekeeping is the lack of engagement between the peacekeeping officials and the local populace. As Séverine Autesserre outlines in a 2015 Foreign Policy article, this creates an environment where the peacekeeping officials develop plans to 'keep' the peace, but they are disconnected from reality, having the opposite effect on the ground. Additionally, it creates a reinforcement mechanism for the peacekeeping officials, because the officials on the ground report that their plan was successfully implemented, but, in reality, it had adverse effects. If the situation on the ground turns into another outbreak of violence, the local populace will be blamed.
This criticism is similar to the critic levelled at development in developing countries by authors such as James C. Scott, James Ferguson, and L. Lohman. Although peacekeeping and development are two different things, the logic behind the criticism is the same. The third-party officials-whether they are peacekeepers or agents of development-are isolated from the general populace, believing they know what is best, and refusing to gather information from a ground level. This is not out of maliciousness or imperialism, but out of a legitimate belief that they, as educated officials with access to other experts and who are well versed in development and peacekeeping literature, know what is best.

Proposed reform: 

Brahimi analysis: In response to criticism, particularly of the cases of sexual abuse by peacekeepers, the UN has taken steps toward reforming its operations. The Brahimi Report was the first of many steps to recap former peacekeeping missions, isolate flaws, and take steps to patch these mistakes to ensure the efficiency of future peacekeeping missions. The UN has vowed to continue to put these practices into effect when performing peacekeeping operations in the future. The technocratic aspects of the reform process have been continued and revitalised by the DPKO in its "Peace Operations 2010" reform agenda. This included an increase in personnel, the harmonisation of the conditions of service of field and headquarters staff, the development of guidelines and standard operating procedures, and improving the partnership arrangement between the Department of Peacekeeping Operations (DPKO) and the United Nations Development Programme (UNDP), African Union, and European Union. A 2008 capstone doctrine entitled "United Nations Peacekeeping Operations: Principles and Guidelines" incorporates and builds on the Brahimi analysis.
One of the main issues that the Brahimi report identifies is the lack of coordination and planning of the Peacekeeping Operations. Also, the difference between the objectives of the Peacekeeping Operations and the resources destined to fund the missions. Therefore, the report asks the Security Council to make clear the goals and the resources to accomplish them.  According to Fearon and Laitin, the Brahimi Report provides a political instrument for the secretary-general to negotiate with the Security Council the goals, the troops, and the resources need it to the operations. This instrument tries to avoid the cases of underfunding presented in Missions such as in Bosnia, Somalia, and Sierra Leone.
Christine Gray analyses the issues of implementing the recommendations of the Brahimi Report. She explains the difficulty in implementing these recommendations. In particular, in reducing the gap between the mandates of Security Council and the actual resources devoted to implementing them.

See also: Bangladesh UN Peacekeeping Force
Canadian peacekeeping
List of United Nations peacekeeping missions
List of countries by number of UN peacekeepers
List of countries where UN peacekeepers are currently deployed
List of non-UN peacekeeping missions
Multinational Force and Observers
Military operations other than war
Temporary International Presence in Hebron
Timeline of UN peacekeeping missions

References: 

Further reading: Aoi, Chiyuki;  Coning, Cedric de;  Thakur, Ramesh (eds.) Unintended consequences of peacekeeping operations. United Nations University, 2007.
Autesserre, Séverine. 2014. Peaceland: Conflict Resolution and the Everyday Politics of International Intervention. Cambridge University Press. ISBN 9781107632042
Barnett, Michael. 2002. Eyewitness to a Genocide: the United Nations and Rwanda. Cornell University Press.
Bastaki, Basil; Staniland, Paul; Popoola, Bryan (2024). "Stabilizing Civil Wars without Peacekeeping: Evidence from South Asia". International Security. 49 (1): 133–170. doi:10.1162/isec_a_00489.