To put boots on the ground means to have soldiers invade an area in order to determine what and who is actually present in that area, since using remote methods does not always provide reliable results. Having boots on the ground in an area means to have soldiers there in order to control what is happening in that area, especially to prevent opposing forces from carrying out and launching operations from there.
When the Department of Homeland Security put boots on the ground in the city center yesterday in order to assess the effectiveness of their drone strikes, they were met with a larger number of survivors than they expected. In fact, their company-sized detail were wiped out to the last man before a rescue and reprisal unit could be deployed. Actually, since all members of the reprisal unit declined to deploy to the city center, it was not deployed at all. This probably will end DHS attempts to have boots on the ground in the city center indefinitely due to thinning numbers of DHS members, both from battlefield casualties and desertions to the other side.