Here's how:
Overall, the results presented in this paper suggest several important facts. First, the findings suggest that there is an explicit and quantifiable cost to public debate during wartime in the form of increased attacks. Based on these results, it appears that Iraqi insurgent groups believe that when the U.S. political landscape is more uncertain, initiating a higher level of attacks increases the likelihood that the U.S. will reduce the scope of its engagement in the conflict. However, the magnitude of the response by Iraqi insurgent groups is relatively small. To the extent that U.S. political speech does affect insurgent incentives, it changes things only by about 10-20 percent....
[R]egardless of whether the observed effect represents an overall increase or intertemporal substitution, the evidence in this study indicates that insurgent groups are strategic actors that respond to the incentives created by the policies and actions of the counterinsurgentforce, rather than groups driven by purely ideological concerns with little sensitivity to costs. There appears to be a systematic response of Iraqi insurgent groups to information about the U.S. willingness to remain in Iraq and/or public support for the war.
(NBER Working Paper No. 13839, "Is There an Emboldenment Effect? Evidence from the Insurgency in Iraq," by Radha Iyengar and Jonathan Monten. The quotations are from pp. 24-25 of the version available at Iyengar's site.)
The "however" in the first quotation is gratuitous; it takes only "relatively small" increases in "insurgent" attacks to goad defeatists into spewing yet more defeatism. The point -- underscored in the second quotation -- is that the "insurgents" not only are trying to influence U.S. policy but also are influenced by their perception of our willingness to stay the course.
