Peter Feaver, a Prof of Political Science at Duke and a Naval Reserve officer, has done a lot of survey research on American public attitudes toward military interventions. Summarizing much of that, Feaver found that the American public is more than willing to sustain casualties in a conflict that they believe is important and that is being prosecuted effectively. Feaver's research tends to support the overly simplistic, but accurate in practical terms, 1/3 and 2 year rules (of thumb). But note that the public is willing to support a war longer and more willingly if the two stated conditions obtain.

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JohnT