Foreign military intervention affects the duration of civil wars (Elbadawi and Sambanis 2000; Linebarger and Enterline 2016; Paquin and Saideman 2017), but precisely how it does so remains an open question. Balch-Lindsay and Enterline (2000) claim that external intervention shortens civil wars whenever the interveners favor one side or the other, but prolongs the fighting if outside states back rival combatants (see also Balch-Lindsay et al. 2008). Regan (2002) likewise argues that foreign intervention in support of one side or the other accompanies shorter wars, yet he also shows that external involvement lengthens the conflict whenever it favors none of the warring parties. Regan and Aydin (2006), by contrast, find that foreign military intervention is associated with longer civil wars, whereas diplomatic initiatives undertaken by external actors effectively reduce the duration of internal warfare.
Cunningham (2010) makes a notable contribution to this research program by proposing that civil wars persist whenever outsiders inject extraneous goals into an ongoing conflict. It is often the case that ‘external states intervene to pursue independent objectives in the war outside of the goals of the domestic combatants. These states, then, fight to advance those (largely self-interested) objectives, not necessarily to help one side win or to help resolve the conflict’ at hand (Ibid.: 116). The introduction of such independent objectives makes it ‘more difficult [to bring the fighting to an end,] because there is [now] an additional actor who has to be either defeated militarily or consent to an agreement to end the war’ (Ibid.: 117). External intervention can, therefore, be expected to accompany lengthier civil wars.
Cunningham’s (2010: 122–124) sophisticated statistical analysis confirms the positive correlation between foreign military intervention and prolonged civil wars, but the proposed explanation for this finding conflates two analytically distinct lines of argument.
The first hypothesis—and the one that Cunningham (Ibid.: 118–119) emphasizes—is that any single intervening the state is likely to bring to the fighting a ‘separate agenda’ that is out of step with the preferences or interests of local combatants, thereby making a negotiated settlement harder to achieve.
The second is that whenever multiple states intervene, their respective priorities and ambitions will tend to clash with one another as well as with those of local actors, making a mutually acceptable resolution more elusive (Ibid.: 117). In order to tease these divergent explanatory accounts apart, it would be useful to know to what extent single-state interventions differ from multiple-state interventions.
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