Molecular dynamics simulations of nanocrystalline FCC metals revealed the strain rate dependence to be reflected in the critical resolved shear stresses for dislocation propagation. The strain rate and temperature dependence study is complemented by a stress and grain size dependence supporting consistently the importance of thermally activated dislocation mediated plasticity in the inverse Hall-Petch regime. A model is presented demonstrating that the strongest size can as well be explained on the sole basis of dislocation mechanisms.