During WW2 the US government purchased a large number of passenger ships and freighters, and "drafted" even more.
The vast majority of these were placed under US Army control, but continued to operate with their previous civilian crews, plus a few Army senior enlisted, warrant officers, and officers as supervisory & control crews. These moved large amounts of personnel and cargo between "secure" ports.
This freed the USN's "combat transports" for actual work to/from/within combat zones.
However, a considerable number of landing craft that were assigned to invasions and support for combat operations (especially in the Pacific) were owned/operated by the US Army... alongside identical vessels in the USN.