Here's a passage from the Army's field manual on counterinsurgency:

Of the preceding characteristics, knowledge of objectives, motivations, and means of generating popular support/tolerance will often be the most important intelligence requirements and the most difficult to ascertain. In particular, generating popular support/tolerance often has the greatest impact on the insurgency’s long-term effectiveness. This is usually the center of gravity of an insurgency.

Center of gravity, it should be said, has a technical meaning in military circles derived from Clausewitz. The enemy's center of gravity is the thing you need to change to win the war. If the support of the population is an insurgency's center of gravity, then they key metric by which you want to measure a counterinsurgent's success is the counterinsurgent's impact on the attitude of the local population.

Here's Michael O'Hanlon and Kenneth Pollack trying to clarify exactly what they did on their July trip to Iraq: "Although we were able to meet with several dozen Iraqi civilians—from people on the streets to local shaykhs—because we were nearly always in the presence of American military personnel, we felt we had little ability to gauge the mood of the Iraqi people."