Manpower and Counterinsurgency
Shawn Robert Woodford
How many troops are needed to defeat an insurgency? This deceptively innocuous question is laden with political and military peril. The answer is, of course, that it depends. Some contend that a precise ratio of counterinsurgents to insurgents will lead to victory, while others argue that force strengths are irrelevant to success or failure. In the wake of America’s decidedly ambivalent experiences in Iraq and Afghanistan, the debate remains unresolved. There have been several recent analytical efforts focused on addressing the question. They generally agree that there is indeed a positive relationship between manpower and outcome in insurgencies. Despite the promise of using empirical research to address the complexities in counterinsurgency, however, this effort is languishing due to a lack of attention and resourcing.
It is not clear where the notion originated that force ratios mattered in counterinsurgency. It appears to have become an article of faith during the British campaign in Malaya, although Sir Robert Thompson — a British staff officer in that conflict and a counterinsurgency theorist influential during the Vietnam War — subsequently claimed that the rule-of-thumb that 10 , 12, or 20 troops were needed per 1,000 insurgents to succeed was the invention of a journalist. The relevance of force ratios made a comeback in the mid-1990s, albeit measured in a different way. Seeking a way of projecting force requirements for sustained stability operations, RAND analyst James T. Quinlivan looked at 13 historical cases and concluded that success required 20 troops were needed per 1,000 local inhabitants. Defining ratios by the number of troops per population became known as force density.
The political volatility of force requirements became vividly apparent during the lead up to the U.S. invasion of Iraq in 2003. U.S. Army Chief of Staff General Eric Shinseki told Congress that “several hundred thousand” American troops would be necessary to stabilize Iraq following the fall of Saddam Hussein. This drew a quick rebuttal from Deputy Secretary of Defense Paul Wolfowitz, who testified that Shinseki’s estimate was “wildly off the mark.” The public disagreement led to Shinseki’s premature retirement and replacement as Army Chief of Staff.
Despite Operation IRAQI FREEDOM’s initial success, the subsequent civil chaos and insurgency appeared to validate Quinlivan and Shinseki. The authors of the U.S. Army’s FM 3-24 Counterinsurgency, revised in 2006, certainly thought so. The manual rejected tie-down ratios but endorsed Quinlivan’s force density concept. Interestingly enough, however, this endorsement was qualified in the subsequent joint doctrinal manual, JP 3-24 Counterinsurgency Operations, published just three years later in 2009. While force density was still considered a useful measure, mention of the specific 20/1,000 ratio was omitted. When JP 3-24 and FM 3-24 were revised again in 2013 and 2014, respectively, neither contained any references to force level benchmarks, tie-down ratios, or troop density. As quickly as the concepts had been adopted, they were abandoned.
Recent Empirical Research
Even as Quinliven’s assertion of the importance of force density became axiomatic during the dark days of the U.S. war in Iraq, other researchers took a closer look at the role force strength played in past insurgencies. Large data sets, such as those provided by the Correlates of War Project or the Empirical Studies of Conflict Project were available but did not contain the detailed strength data needed for force ratio analysis. The urgency of the Iraq conflict led to U.S. and British government funding to facilitate new data collection and large-N analytical studies.
The Dupuy Institute (TDI)
Sponsored by various U.S. government agencies, TDI amassed data on 109 post-World War II insurgencies, interventions, and peacekeeping operations between 2004 and 2009. Led by Chris Lawrence, TDI’s analytical effort found that insurgency outcomes closely tracked the force ratio of counterinsurgents to insurgents. While overwhelming numbers were not required to defeat an insurgency, force ratios above 10-to-1 nearly always produced a counterinsurgent victory. Conversely, lower force ratios did not preclude success, but conflicts with two or fewer counterinsurgents per insurgent greatly favored an insurgent victory.
When force ratios were assessed together with the nature of the motivation for the insurgency, TDI found that as long as counterinsurgents had favorable odds, they did not have a major impact on the outcome of insurgencies with a limited political or ideological basis. However, when facing broadly popular insurgencies, counterinsurgents lost every time they possessed a force ratio advantage of 5-1 or less, failed half the time with odds between 6-1 and 10-1, but succeeded three-quarters of the time when outnumbering the insurgents by 10-1 or more.
The British Ministry of Defence initiated a three-year research program by Dstl in 2004, which collected data on 44 post-World War I asymmetric conflicts. A study led by Andrew Hossack analysing 34 of those cases concluded that there is a relationship, albeit weak, between between the odds of military campaign success and force ratios as measured in terms of a ratio of median annual force strengths. The advantages were relatively modest. Each tenfold increase in the ratio of counterinsurgents to insurgents improved the chances of success by only 30%. A 10-1 manpower advantage yielded a 47% chance of success. Improving the ratio to 100-1 still only afforded a 77% chance of winning.
For a RAND study of insurgency and counterinsurgency in 2006, Martin Libicki compiled data on 89 post-World War II insurgencies from a dataset of 127 insurgencies created by James D. Fearon and David D. Laitin. With regard to force ratios, Libicki concluded that “as a general rule, the greater the government-to-insurgent force ratio, the lower the odds of an insurgent victory,” but that “the relationship is not particularly strong.”
In 2008, CAA contracted TDI to convert its post-World War II insurgency research into the Irregular Warfare Database containing 102 cases. In 2009, CAA’s Justine Blaho and Lisa Kaiser used the data to model the outcome of irregular wars. They tested 34 independent variables from 74 cases and found 11 with significant correlations to outcome, including peak year counterinsurgent-to-insurgent ratio. The variable for counterinsurgent-per-civilian (force density) was removed because it did not demonstrate a significant correlation to outcome. Among other conclusions, they determined that counterinsurgents had a greater probability of winning an irregular war if the peak year counterinsurgent-to-insurgent ratio was high.
Another CAA study in 2009 sought to determine the number of counterinsurgent forces required to arrest and reverse a given level of insurgent violence using 42 cases from the Irregular Warfare Database. CAA’s analysts rejected the validity of counterinsurgent to insurgent force ratios citing the unreliability of data on insurgent force size, insurgent and civilian casualties, and counts of insurgent attacks. Instead, they adopted the force density construct “in accordance with doctrine and previous studies,” concluding that “the minimum counterinsurgent force is 2.8 soldiers per 1,000 residents, with more forces required as the violence level increases.”
As part of the 2010 Quadrennial Defense Review, IDA analysts studied 41 cases from the CAA Irregular Warfare Database to try to establish a relationship between force density and outcomes. They too deemed force ratios to be unreliable because “information about the size of insurgent forces is insufficiently credible to be subjected to statistical analysis.” Acknowledging previous studies that did not find a statistically meaningful link between force density and outcome, the IDA analysts adjusted the dataset to examine force densities in localized counterinsurgent areas of operation and to re-score the outcomes of several cases in the database. Using this adjusted data, the IDA analysts found a statistically significant, though not robust, relationship between force density and outcome. They concluded that a density of 20‐25 troops per thousand population was consistent with a 50‐60% chance of success. Successful outcomes decreased greatly for force densities below 15 troops per 1,000, but for operations with ratios above 40 troops per 1,000, the success rate exceeded 75%.
In 2011, Friedman looked into the question of how manpower affects counterinsurgency. Starting with a dataset created by Jason Lyall and Isaiah Wilson III of 173 post-World War I insurgencies, Friedman added his own research on counterinsurgent force strengths, area and population, and insurgent force size. He concluded that there was no statistical correlation between outcome and overall numbers of counterinsurgents or counterinsurgents per area. He found no empirical support for Quinliven’s 20/1,000 ratio. Force ratios were significant only with controls on the data. However, troop density – measured as the ratio of counterinsurgents per inhabitant in a specific area of operations– demonstrated a consistent, positive correlation with counterinsurgent success. Manpower did not necessarily have a decisive impact on insurgency outcome, however. A troop density of 80 counterinsurgents per 1,000 inhabitants was less than twice as likely to succeed as a force sized an order of magnitude lower. In a model derived from his analysis, Friedman observed that as force size increased from 5 to 80 troops per 1,000 inhabitants in the area of operations, the probability of success only rose by less than 15 percent.
At first glance, it would appear that the debate over the relationship of manpower to counterinsurgency outcomes remains an open one. With regard to force density and insurgency outcome, CAA asserted that a statistically meaningful relationship existed, while Dstl and Blaho & Kaiser concluded that none could be established. IDA and Friedman found a statistically meaningful relationship between force density in defined areas of operation and insurgency outcome.
Dstl, RAND, Blaho & Kaiser, and TDI discerned a statistically meaningful relationship between force ratios and insurgency outcome. TDI found a strong relationship between force ratios, the nature of an insurgency, and insurgency outcome. Friedman identified a statistically meaningful relationship between force ratios and outcome when controls were applied to the data. CAA and IDA, however rejected the validity of force ratios, alleging the relevant data to be inherently unreliable.
There appears to be little statistical support for the original Quinlivan troop density construct as measured by the number of counterinsurgents per inhabitant. The only study to support this without qualification was CAA. The extensive data collection and testing conducted by Friedman cast serious doubt on the validity of force level benchmarks.
IDA and Friedman both made a case for the usefulness of troop density as measured by the number of counterinsurgents per inhabitant within a defined area of counterinsurgent operations. When measured in this manner, a clear correlation was found to exist between troop density and insurgency outcome. This notion has considerable qualitative appeal. Insurgencies generally do not occur uniformly throughout an entire country, but rather within specific regions or areas. Consequently, counterinsurgent forces are not deployed uniformly throughout a country, but are concentrated in the places with the highest insurgent activity. This revised troop density construct merits further study.
The argument against using force ratios is that imprecise estimates or unreliable sources supposedly invalidate insurgent strength data for analysis. This claim seems overstated. Data supporting a 10-to-1 ratio would have to be inaccurate by a factor or 10 or greater to nullify the results. If all the insurgent data was off by a factor of 2, for example, it would only change the ratios, not the demonstrated cause and effect relationship. In nearly all the datasets, the insurgent data are collected as reported, which augers against systematic bias. Insurgent data may be noisy, but that is an insufficient reason alone to discount it.
Another possibility for the lingering disagreements could be variations in definitions of terms and variables. It should be noted that despite the very large body of research and scholarship on insurgency and counterinsurgency, there is no consensus on how to define such conflicts. Both IDA and Friedman pointed out that analytical outcomes are sensitive to how the variables are defined. The potential implications of this should be examined more thoroughly.
More Work Is Needed
Amidst the disagreement, however, one salient conclusion stands out: all of the studies generally agreed that there was a positive correlation between counterinsurgent force strength and the outcome of an insurgency. The collective analysis suggests that the commitment of larger numbers of counterinsurgent forces has historically correlated with more successful counterinsurgency campaign outcomes. What remains open to dispute is just how significant this finding is. More data collection, research, and analysis is clearly needed to follow up these promising initial lines of inquiry.
Disagreement over force ratio and force density measures may not be as relevant as it might appear. More analysis is needed to examine how decisive manpower advantages may be. TDI concluded that a force ratio advantage can be decisive against insurgencies with broad popular support, while Dstl and CAA suggested that a counterinsurgent manpower advantage may be important largely to prevent insurgent military success. Dstl and Friedman indicated that there may be points of diminishing manpower returns. Given the potential difficulties in generating significant additional counterinsurgent manpower, an advantage may be applicable and useful only under particular circumstances. More data and analysis is needed to verify the validity of the postulated relationship between counterinsurgent force levels and the local population within an active area of operation.
Due to the limitations of the available data, all of the studies based their analysis on data averages. The figures used for insurgent and counterinsurgent force sizes were usually selected from the highest annual totals across years or decades. All of the studies indicated the need to obtain more detailed data on individual cases to allow for more discreet and dynamic analysis to look for undetected links and patterns. TDI in particular has called for examination of conditions before insurgencies begin and when they are just getting underway.
Retired General Robert Scales recently lamented how the U.S. military abandoned the experience it had gained during the Vietnam War and expressed fear that it was doing so again with hard-won knowledge from Afghanistan and Iraq. Research and analysis on counterinsurgency was left to languish after Vietnam, only to be exhumed under the dire circumstances of the U.S. war in Iraq. The last U.S. government sponsored study on manpower in counterinsurgency ended in 2010, and Friedman published his findings in 2011. Major James Zanella surveyed existing methods for assessing combat power in irregular warfare for his 2012 U.S. Army School of Advanced Military Studies thesis and concluded “that current Army doctrine does not provide a coherent theoretical method for determining force density requirements in contemporary operating environments.” In the wake of another round of irregular conflicts with dissatisfying endings, important questions remain unanswered. It would be deeply unfortunate if this particularly bit of history was allowed to repeat itself.