The unified reaction valley approach (URVA) was used to investigate the mechanism of the reaction between ethene and 1,3-butadiene. The reaction valley was explored using different methods (Hartree-Fock, second-order Moller-Plesset perturbation theory, density functional theory (B3LYP), coupled cluster theory (CCSD-(T)), and the basis sets 3-21G, 6-31G(d), and 6-311G(d,p). Results were analyzed by characterizing normal modes, the reaction path vector, and the curvature vector in terms of generalized adiabatic modes associated with internal parameters that are used to describe the reaction complex. The Diets-Alder reaction possesses three transition states (TS), where TS1 and TS3 correspond to bifurcation points of the reaction path, at which a C-s-symmetrical reaction complex converts into two C-2-symmetrical reaction complexes. The activation enthalpy at 298 K is 23.5 kcal/mol according to calculations and corrected experimental data. It is determined by a symmetry-supported charge-transfer step, which is followed by a spin-recoupling and a bond formation step. Energy dissipation is strong in the exit channel. Mode selective rate enhancement seems to be not possible for symmetry-allowed reactions, which are characterized by a collective change of many internal coordinates of the reaction complex. Manipulation of the reaction barrier or the reaction mechanism (from concerted to nonconcerted) is only possible in the charge-transfer step. Contrary to opposite claims, aromaticity plays only a minor role for the TS energy.