An unexpected exodus of Baltimore police officers in the days and weeks before cuts to their pension benefits took effect has left the department short-staffed at a critical juncture in its efforts to reduce crime.

Officers have complained for months that the patrol division is not fully staffed, a contention that department commanders had called untrue. But for the first time, the city’s police commissioner is now warning that “staffing is a huge problem” and that shortages could risk breaking momentum in curtailing homicides, shootings and other crime.