World War II, as the name implies, was a tremendous conflict with gigantic leaps forward in both technologies and the tactics that went along with them. As a result of the conflict, infantry-troops carried, shoulder-fired semi-automatic firearms became the norm, first inspired by the fielding of the Garand and later perfected by the Nazis and Soviets.
As the powers stepped back from the war and looked to learn from the conflict, it was apparent that the use of full-powered ammunition was “overkill” for the fielding of combined arms. The United States immediately started work on what would come to be the 7.62×51 NATO round (based on the .308 Winchester) and the M-14.
However, the United Kingdom took a different path. Not encumbered by the firmly held belief that long range fire and an understanding that the conflicts of tomorrow would no longer be defined solely by open battlefields, the British developed two technologies that might have (and many would argue, should have) been: the .280 British and the EM-2 bullpup rifle.
As the US Military grapples today with a capability gap, the British then recognized the true value of an intermediate cartridge with solid ballistics to engage targets out to the practical effective distances of shoulder-fired weapons combat. However, the US was entrenched in the .30 caliber (an attitude which still persist today), which lead to some conflict between the friendly nations.
Ultimately, Winston Churchill pushed the UK to adopt the 7.62×51 cartridge to appease the United States, but only after Canada had agreed to adopt the .280 if the US did too.