Dee ihe ndị ọzọ n'isiokwu a - Soviet Union wakporo Poland na Septemba 1939 n'okpuru Molotov-Ribbentrop Pact na Germany; Soviet Union wakporo Finland
na November 1939; na June 1941, Germany kagburu nkwekọrịta ịghara ịwakpo mba ọzọ ma wakpo Soviet Union. Ọ bụ ezie na USSR nwere ndị ọkà mmụta sayensị na ndị injinia pụtara ìhè, malitere nyocha na ihe ga-emecha bụrụ radar ("radiolokatsiya", lit. radiolocation) ozugbo onye ọ bụla ọzọ, ma nwee ọganihu dị mma na mmepe magnetron mbụ, ọ banyere agha na-enweghị usoro radar zuru oke, nke nwere ike.