The largest weapon ever made using modern concepts using physics to harness natural forces unknown to earlier societies is a thermonuclear weapon once detonated by the Soviets. In the era after the Americans dropped two atomic bombs on Japan during World War II, the competition between the U.S. and U.S.S.R. sparked a race to build the largest arsenal of nuclear weapons to ensure defense capabilities should a conflict arise. This led to a series of nuclear device tests conducted by both states throughout what would become known as the Cold War, until all atmospheric nuclear tests altogether. Before that, some mighty powerful weapons were detonated, but none so powerful as the Tsar Bomba.
Tensions were extraordinarily high between the two nations in the early 1960s, and the Soviets wanted to project strength. To that end, they created many nuclear weapons, and the largest of them all was detonated on October 30, 1961. It was a three-stage hydrogen bomb which, to simplify, uses an atomic fission bomb to begin the detonation, that, in turn, compresses the next stage and begins the fusion process that turns out a much greater yield. It yielded 50 megatons, although there were alterations made to prevent it from being more. It could have hit 100 if not for substitution of lead for uranium in parts of it as the original design called for.