Carbon nanotubes yield to mechanical force by a primary dislocation dipole whose formation energy describes the thermodynamic stability of the tubule. However, the real-time strength is determined by the rate of defect formation, defined in turn by the activation barrier for the bond flip. First extensive computations of the kinetic barriers for a variety of strain-lattice orientations lead to predictions of the yield strength. Its value depends on nanotube chiral symmetry, in a way very different from the thermodynamic assessment.