Peacekeeping
Effectiveness of peacekeeping missions: Reviews of the academic literature show considerable evidence that peacekeeping increases peace. According to Fortna, there is strong evidence that the presence of peacekeepers significantly reduces the risk of renewed warfare; more peacekeeping troops contribute to fewer battlefield deaths; and more peacekeeping troops contribute to fewer civilian deaths. A study by political scientists at Uppsala University and Peace Research Institute Oslo estimates that an ambitious UN peacekeeping policy with a doubled peacekeeping operation and strong mandates would "reduce the global incidence of armed conflict by two thirds relative to a no-PKO scenario." According to Fordham University political scientist Anjali Dayal, "Scholars have found that peacekeeping keeps wars from bleeding across borders. Having more peacekeepers on the ground also seems to correspond with fewer civilians targeted with violence. And peace operations at times have successfully served as transitional authorities, handing power back to local authorities, although this is decreasingly true." A 2018 study found that peacekeeping reduces the severity of civil war on its own, but when it is coupled with mediation, the impact is greater. There is also evidence that the promise to deploy peacekeepers can help international organizations bring combatants to the negotiation table and increase the likelihood that they will agree to a cease-fire.
By controlling for specific factors that affect where peacekeepers are deployed and what the potential chances for peace are, Page Fortna's statistical research shows that there is a statistically significant impact on lasting peace when peacekeepers are deployed. Despite the fact that peacekeepers are sent to locations where peace is least likely to succeed, Fortna finds that conservative estimates suggest that the presence of UN peacekeepers diminishes the risk for renewed violence by at least 55%-60%; with less conservative estimates upwards of 75%-85%. Additionally, her analysis concludes that there is little difference in the effectiveness between Chapter VI consent-based missions and Chapter VII enforcement missions. Indeed, enforcement missions only remain effective if the UN peacekeeping force can prove and sustain their credibility in the use of force. This stresses the importance of a UN mission maintaining the consent of the peacekept. Ultimately, Fortna finds that peacekeeping is an effective tool for ensuring a lasting peace; especially compared to situations in which belligerents' are left to their own devices. Utilising the previously mentioned causal mechanisms for peacekeeping, a UN peacekeeping force can have a substantial and substantive impact on sustaining a lasting peace. Having a relative consensus of the positive impact of peacekeeping for ensuring a lasting peace, Fortna and Howard suggest that  the literature is moving towards the study of  i) the effectiveness of the types of peace-keepers, ii) the transitional administrations, iii) the links between peacekeeping and democratisation, and iv) the perspectives of the "peacekept".
Doyle and Sambanis' analysis finds that lasting peace is more likely after non-ethnic wars in countries with a relatively high level of development in addition to whether or not UN peacekeeping forces and financial assistance are available. They conclude that in the short run lasting peace is more dependent on a robust UN deployment coupled with low levels of hostility between belligerents. They note that increased economic capacity can provide an incentive not to renew hostilities. In the long run, however, economic capacity matters far more whereas the degree of hostility between belligerents is less important. As successful as UN deployments can be, they have inadequately spurred independent economic development within the countries where they have intervened. Thus, the UN plays a strong, but indirect role and success in lasting peace is predicated on the development of institutions that support peace, rather than serving as a deterrent for renewed war.
Other scholarly analyses show varying success rates for peacekeeping missions, with estimate ranging from 31 percent to 85 percent.
According to a 2020 study, non-UN peacekeeping missions are as effective as UN peacekeeping missions. Another 2020 study found that peacekeeping successfully protected civilians.
A 2021 study in the American Journal of Political Science found that UN peacekeeping in South Sudan had a positive impact on the local economy.

Factors that impact lasting peace: There are many factors that can have a negative impact on lasting peace such as hidden information about the relative strength possessed by the belligerents; a rebel group's involvement in illicit financing through means such as through the export of diamonds and other minerals; participation in the trafficking of drugs, weapons and human beings; whether or not military victory was achieved by one side; the length of the war as well as how costly it was; commitment problems and security dilemma spirals experienced by both sides; whether a cease-fire or treaty signed by the belligerents; lack of transparency in the motives and actions carried out by belligerents in the immediate aftermath of the conflict; extremist spoilers; participants in the conflict that may benefit from its continuation; indivisibility and more.
Perhaps one of the most statistically significant contributors to a lasting peace is whether or not military victory was achieved by one side. According to Fortna's research, civil wars in which one side wins, resulting in a cease-fire or truce, have an approximately 85–90% lower chance of renewed war. Moreover, peace treaties further reduce the risk by 60–70%.
If a group is funded by drugs, diamonds or other illicit trade then there is a substantial increase in the chance of renewed violence—100–250%—which is to say that in such circumstances war is two to three-and-a-half times more likely to begin again. While Fortna finds that wars which involve many factions are less likely to resume, Doyle and Sambanis find the opposite.
Costly wars and wars fought along identity lines both provide varied chances of the renewal of violence. While longer wars and peace established by treaty (especially those attained by military victory) can reduce the chances of another war.

Impacts of peacekeeping on participating forces: 

Military normalisation: Some commentators have highlighted the potential to leverage peacekeeping operations as a mechanism for advancing military normalisation. Michael Edward Walsh and Jeremy Taylor have argued that Japan's peacekeeping operations in South Sudan provide those promoting Japan's military normalisation with "a unique opportunity to further erode the country's pacifist constitution." "Unable to accept the full weight of modern peacekeeping operations without fundamental political, legal, and social changes," they conclude that "Japan's peacekeepers remain ill-prepared to tackle many serious contingencies requiring use of deadly force." For this reason, they suggest that Japan's continued participation in UN peacekeeping operations might force policy changes that ultimately push the country toward "a tipping point from which the normalisation of Japan's military (will be) the only outcome."

Political impact on sending countries: Diana Muir Appelbaum has expressed concern that the creation of a military in Fiji for the purpose of serving in international peacekeeping missions, has produced a military powerful enough to stage four coups d'état (1987, 1999–2000, 2006, and 2009) and to rule Fiji as a military dictatorship for over two decades. However, a 2018 study published in the Journal of Peace Research found that countries where militaries are highly dependent on the funds they receive from UN peacekeeping were less likely to experience coups d'états than comparable countries less dependent on such funds.

Impacts on individual peacekeepers: Studies of peacekeeping soldiers show both positive and negative effects. A study of 951 US Army soldiers assigned to Bosnia revealed that 77% reported some positive consequences, 63% reported a negative consequence, and 47% reported both.  The peacekeepers are exposed to danger caused by the warring parties and often in an unfamiliar climate. This gives rise to different mental health problems, suicide, and substance abuse as shown by the percentage of former peacekeepers with those problems. Having a parent in a mission abroad for an extended period is also stressful to the peacekeepers' families.
Another viewpoint raises the problem that the peacekeeping may soften the troops and erode their combat ability, as the mission profile of a peacekeeping contingent is totally different from the profile of a unit fighting an all-out war.

Criticism: 

Peacekeeping, human trafficking, and forced prostitution: Since the 1990s, UN peacekeepers have been the subject of numerous accusations of abuse ranging from rape and sexual assault, to pedophilia and human trafficking. Complaints have arisen from Cambodia, East Timor and West Africa. In Bosnia-Herzegovina prostitution associated with trafficked women skyrocketed and often operated just beyond the gates of U.N. compounds. David Lamb, a regional human rights officer in Bosnia from 2000 to 2001 claimed "The sex slave trade in Bosnia largely exists because of the U.N. peacekeeping operation. Without the peacekeeping presence, there would have been little or no forced prostitution in Bosnia." In addition, hearing held by the U.S. House of Representatives in 2002 found that members of SFOR were frequenting Bosnian brothels and engaging in sex with trafficked women and underage girls.
Reporters witnessed a rapid increase in prostitution in Cambodia, Mozambique, Bosnia, and Kosovo after UN and, in the case of the latter two, NATO peacekeeping forces moved in. In the 1996 UN study called "The Impact of Armed Conflict on Children", former first lady of Mozambique Graça Machel documented: "In 6 out of 12 country studies on sexual exploitation of children in situations of armed conflict prepared for the present report, the arrival of peacekeeping troops has been associated with a rapid rise in child prostitution".
Gita Sahgal spoke out in 2004 with regard to the fact that prostitution and sex abuse crops up wherever humanitarian intervention efforts are set up.