While most Americans admire the military and service members, they know very little about our armed forces.

“Most people know nobody in the military,” Jim Mattis, who headed U.S. Central Command before retiring from the Marine Corps as a four-star general in 2013, told Military Times. Mattis is the co-author of a new book that chronicles the cultural gap between the military and the civilian population it serves.

Only half of the thousands of Americans surveyed for the book recalled socializing with a service member or military spouse within the past year. One in three respondents said they had little or no familiarity with the military.

American society’s lack of understanding about the military likely reflects the shrinking size of the armed forces and recruiters’ heavy reliance on specific regions such as the South, according to the story. Five rounds of base closure, which has consolidated personnel in fewer locations, also has contributed to the civilian-military gap.

As long as civil-military relations remain healthy, that gap is not a major problem, Mattis said, but he worries about the consequences of the divide potentially expanding.

“America is quite right to be proud of their military, but at the same time there has got to be a sense of common purpose between these two elements. If, in fact, this gap grows and we lose the sense of common purpose, then I think we have a problem,” he said.