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There is significant uncertainty about the relationships between microstructure, plasticity mechanisms, and strength in high pressure and high strain-rate conditions. Using molecular dynamics simulations of propagating ramp compression and release waves, we examine strength and plasticity in [100] single crystalline and nanocrystalline tantalum. We find that the strength response of Ta under dynamic loading can be separated into two categories: the elastic/plastic transition and steady plastic flow regimes. The single crystalline and nanocrystalline Ta display significantly different strengths in the elastic/plastic transition regime. Surprisingly, the two forms of Ta exhibit similar strengths in the steady plastic flow regime despite their differences in microstructure and plastic deformation mechanisms. We also find that the transition from the elastic/plastic regime to the steady plastic flow regime is accompanied by either dislocation density reaching saturation or twin nucleation and growth. Twinning only occurs when the dislocation density has not reached saturation before transitioning into the steady plastic flow regime. The nucleated twins that formed during the transition to steady plastic flow shrink and disappear as dislocation density increases to its saturation value. [ABSTRACT FROM AUTHOR]