The use of standardized steel shipping containers began during the late 1940s and early 1950s, when commercial shipping operators and the U.S. military started developing such units.

     During World War II, the US Army began experiments with containers to ship supplies to the front lines. Cargo was being delayed at ports due to the time required by break bulk loading and offloading of ships. In addition the supplies suffered from pilferage and in-transit damage.

     In 1948 the U.S. Army Transportation Corps developed the "Transporter", a rigid, corrugated steel container, able to carry 9,000 pounds (4,082 kg). It was 8 ft 6 in (2.59 m) long, 6 ft 3 in (1.91 m) wide, and 6 ft 10 in (2.08 m) high, with double doors on one end, was mounted on skids, and had lifting rings on the top four corners.

  After proving successful in Korea, the Transporter was developed into the Container Express (CONEX) box system in late 1952.

     Based on the Transporter, the size and capacity of the CONEX were about the same, but the system was made modular, by the addition of a smaller, half-size unit of 6 ft 3 in (1.91 m) long, 4 ft 3 in (1.30 m) wide and 6 ft 10 ½ in (2.10 m) high.

    CONEXes could be stacked three high, and protected their contents from the elements.

     By 1965 the U.S. military used some 100,000 Conex boxes, and more than 200,000 in 1967, making this the first worldwide application of intermodal containers.

     More than three quarters were shipped only once, because they remained in theatre. The use of CONEX boxes in Vietnam was only limited by the imagination of the troops, and were as useful to the soldiers as their contents.