The soviet exploded its first atomic bomb in 1949, how did the United States react to this?

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 nowehere) 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.