The transition from static to sliding friction is mediated by rapid interfacial ruptures propagating through the solid contacts forming a frictional interface. While propagating, these ruptures correspond to true shear cracks. Frictional sliding is initiated only when a rupture traverses the entire interface; however, arrested ruptures can occur at applied shears far below the transition to frictional motion. Here we show, by measuring the real contact area and strain fields near rough frictional interfaces, that fracture mechanics quantitatively describe rupture arrest and therefore determine the onset of overall frictional sliding. Our measurements reveal both the local dissipation and the global elastic energy released by the rupture. The balance of these quantities entirely determines rupture lengths, whether finite or system-wide. These results confirm a fracture-mechanics-based paradigm for describing frictional motion and shed light on the selection of an earthquake's magnitude.