QCD is a nonabelian gauge theory, for example. The generalized Maxwell action is called the Yang-Mills action. The corresponding equations are very similar, e

QCD is a nonabelian gauge theory, for example. The generalized Maxwell action is called the Yang-Mills action. The corresponding equations are very similar, except the gauge-covariant derivatives do not take a particularly simple form (which they do in the U(1) case because U(1) is abelian). https://physicstravelguide.com/equations/yang_mills_equations#tab__why_is_it_interesting https://physicstravelguide.com/equations/yang_mills_equation...