When there is no movement in a tug of war, the net force on the system of the two participating teams, plus the rope they're pulling, is zero.

This statement is an application of Newton's first law of motion, aka the "law of inertia." There are two forces exerted in opposite directions, each team pulling on one end of the rope. When the pulling forces from each team are equal in magnitude, the net force is zero, leading to the equilibrium state.