Strauss, however, deeply distrusted Oppenheimer. He had become aware of Oppenheimer's former Communist affiliations before World War II and questionable behavior during the war, and he began to think that Oppenheimer might even be a Soviet spy. Strauss was suspicious of Oppenheimer's tendency to downplay Soviet capabilities. In 1953, Oppenheimer stated in the July edition of "Foreign Affairs" that he believed the Soviets were "about four years behind" in nuclear weapons development. The United States had exploded the first thermonuclear device the previous year; however, only a month after Oppenheimer made his proclamation, in August 1953, the Soviet Union declared that it had tested its own fusion-based bomb, which U.S. sensors identified as a boosted fission weapon. Strauss was not alone in having his doubts; a number of other officials in Washington were also suspicious that Oppenheimer might be a security risk.