Like, WW2 convoys? Hmm. I don't think that 20/40ft containers were used in WW2. I remember footage of people loading ships using pre-containerized stuff.

googles

Ah, after that, Korean War. It also sounds like the US military's use did a lot to promote the use of a standard container size, but theirs was significantly smaller, and it wasn't the first attempt.

https://en.wikipedia.org/wiki/Intermodal_container

The use of standardized steel shipping containers began during the late 1940s and early 1950s, when commercial shipping operators and the US military started developing such units.[18] In 1948 the U.S. Army Transportation Corps developed the "Transporter", a rigid, corrugated steel container, able to carry 9,000 pounds (4,100 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.[19] 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,[nb 2] 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+1⁄2 in (2.10 m) high.[22][23][nb 3] CONEXes could be stacked three high, and protected their contents from the elements.[20] By 1965 the US military used some 100,000 Conex boxes, and more than 200,000 in 1967,[22][26] making this the first worldwide application of intermodal containers.[20] Their invention made a major contribution to the globalization of commerce in the second half of the 20th century, dramatically reducing the cost of transporting goods and hence of long-distance trade.[27][28]

https://en.wikipedia.org/wiki/Conex_box

The CONEX box (a portmanteau of "Container, express") is a type of cargo container that was developed during the Korean War and was used to transport and store supplies during the Korean and Vietnam wars. It was reinvented by Malcom McLean to form the standard intermodal shipping container (often called an ISO box, after ISO 668/ISO 6346) that is used widely by container shipping companies today.

The use of standardized steel and aluminum shipping containers began during the late 1940s and early 1950s, when commercial shipping operators and the US military started developing such units.[2] 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.[1] 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.[3]

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,[nb 1] 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+1⁄2 in (2.10 m) high.[6][7][nb 2] CONEXes could be stacked three high, and protected their contents from the elements.[4] By 1965, the US military had some 100,000 CONEX boxes, and by 1967, over 100,000 more had been procured to support the escalation of the Vietnam War,[7][10] making this the world's first intercontinental application of intermodal containers.[4] More than three quarters were shipped only once, because they remained in theatre. The CONEX boxes were as useful to the soldiers as their contents, in particular as storage facilities where there were no other options.[10]

The term "CONEX" remains in common use in the US military to refer to the similar but larger ISO-standard shipping containers.

Then Malcom McLean tried -- successfully -- to introduce containerization at a large scale in the civilian sector:

https://en.wikipedia.org/wiki/Malcom_McLean

Malcolm Purcell McLean (November 14, 1913 – May 25, 2001)[1] was an American businessman who invented the modern intermodal shipping container, which revolutionized transport and international trade in the second half of the twentieth century. Containerization led to a significant reduction in the cost of freight transportation by eliminating the need for repeated handling of individual pieces of cargo, and also improved reliability, reduced cargo theft, and cut inventory costs by shortening transit time. Containerization is a major driver of globalization.

But while there's a direct line of descent from McLean's containers to ISO containers, it still wasn't exactly his size that became the standard:

https://www.inventionandtech.com/content/evolution-box-0

Containerized shipping vastly increased productivity, but it required standardization before it could be widely used. With that goal in mind, McLean made Sea-Land’s patents available royalty-free to the International Standards Organization (ISO). The ISO set the outside dimensions of containers at 10, 20, and 40 feet long, 8 feet wide, and 8 feet high, later adding 8½- and 9-foot heights. Ironically, it did not include McLean’s 35-foot containers, which meant that Sea-Land had to convert a number of its cellular ships.

https://www.icontainers.com/us/2019/08/06/history-of-teu-twenty-foot-equivalent-unit/#:~:text=TEU%20has%20its%20roots%20in,And%20it%20was.

https://en.m.wikipedia.org/wiki/Malcom_McLean