Complex sentence: Johnston spent nearly a month in the U.S.S.R., was the first American diplomat to tour the Central Asian SSRs, and had a three hour long discussion with Stalin at a time when Ambassador W. Averell Harriman had yet to meet the Soviet premier.
Split into simpler sentences: Johnston spent nearly a month in the U.S.S.R., and was the first American diplomat to tour the Central Asian SSRs. He met with Stalin in a three hour long meeting when Ambassador W. Averell Harriman had yet to meet the Soviet premier.