First-principles calculations combined with kinetic Monte Carlo simulations are carried out to unambiguously demonstrate the vital role of van der Waals (vdW) interactions in the self-assembly of styrene nanowires on H-terminated Si(100) surfaces. We find that, only with the inclusion of London dispersion forces, accounting for the attractive parts of vdW interactions, are the effective intermolecular interactions reversed from repulsive to attractive. Such attractive interactions, in turn, ensure the preferred growth of long wires under physically realistic conditions as observed experimentally. We further propose a cooperative scheme, invoking the application of an electric field and the selective creation of Si dangling bonds, to drastically improve the ordered arrangement of the molecular nanowires. The present paper represents a significant step forward in the fundamental understanding and precise control of molecular self-assembly guided by London dispersion forces.