  The global behavior of solutions is described for the smallest chemical
reaction system that exhibits a Hopf bifurcation, discovered in \cite{WH1}.
This three-dimensional system is a competitive system and a monotone cyclic
feedback system. The Poincar\'e-Bendixson theory extends to such systems
\cite{MS,H0,HS,S} and a Bendixson criterion exists to rule out periodic orbits
\cite{LM}.
