Radio was initially fairly cumbersome, so in WW1 it was restricted but was not a long range threat - still all of Europe feared the spy sending data to another spy in another country After WW1 Europe stayed locked down with draconian laws limiting private radios.
At this time they relied on fenced borders - radio made them crap their pants. They had a Spy-Vs-Spy mind set, still it was manageable, long range stuff was large in terms of antennas etc.
The USA relied in the Atlantic ocean - small transmitters did not have the range and large ones were easy to find and hard to hide.
The net USA result was radio amateurs, crystal sets etc were not limited apart from the licence. So radio grew enormously in the USA, thousands of independent radio stations and millions of radio receivers. In Europe radio transmitters were government owned, BBC etc, so it grew a little but nothing like the USA. Rest of Europe more or less similar.
By 1939 = WW2 all in europe were in the middle ages, more or less, in terms of radio at all levels. At universities etc, research took place, but wide dissemination and adoption was interdicted by this oppressive milieu.
Thus the USA gained on the UK/Europe after WW1 and by the time WW2 happened it was far ahead. During WW2 radio was locked down again, but the USA has by far the largest pool of amateur radio people - who all knew Morse Code.