Complex sentence: During the mid- and late-1930s, radio systems for the detection and location of distant targets had been developed under great secrecy in the United States and Great Britain, as well as in several other nations, notably Germany, the USSR, and Japan.
Split into simpler sentences: During the mid- and late-1930s, radio systems for the detection and location of distant targets had been developed under great secrecy in the United States and Great Britain. There were similar developments in several other nations, notably Germany, the USSR, and Japan.