How did the united states react to the soviet union’s development of an atomic bomb in 1949?

It set off an arms race for making nuclear bombs

The US did two major things to react to this. One was to try for a way to ensure nuclear peace. The other was to try to be stronger than the Russians. The first thing that the US did was to propose a plan where both the US and the Soviet Union would turn over all their weapons to the United Nations so that neither nation would be able to use the weapons. But at the same time (especially since that proposal went nowhere) the US started working on bigger bombs -- hydrogen bombs. The US also adopted the ideas of NSC-68, which called for larger conventional forces so as to have an option other than nuclear war.

Hope I helped, 
Ms. Weasley