The strong force is what keeps nuclei together. It is described by a theory called quantum chromodynamics (QCD), which described the forces between fermions called quarks that make up the hadrons. These forces are mediated by spin-1 bosons called gluons. Notice that this is a case where a series in powers of the coupling constant does not make a lot of sense, since higher powers have about the same value as lower powers. Such a theory is called non-perturbative.