In 1954 the US tested an H-bomb in the Marshal Islands. It was a dirty design to be detonated in a ground burst (which maximizes fallout production). The yield was greater than expected and wind patterns changed, contaminating nearby islands and poisoning their inhabitants. The partial test ban treaty was ratified by congress in 1963 but had been in the works since the late '50's. Radiation hazards had a lot to do with it - there was considerable backlash against atmospheric testing after the poisoning of the Marshall Islanders.

The danger to the islanders was known, but it was not until decades later that the extent of the damage became clear. The same applies to the rest of us. The government knew that fallout from nuclear tests spread across the country, but they could not have known at the time what the effects would be. High doses of radiation cause acute radiation poisoning, but the cancer and birth defects caused by lower doses may not become apparent for decades. For that reason, it's only now that the toll of nuclear testing is becoming apparent in studies.