  We consider the impact of the competition among superconductivity, spin
density wave, and nematic order in iron pnictides, and show that the ordering
competition substantially reshapes the global phase diagram. We perform a
detailed renormalization group analysis of an effective field theory of iron
pnictides and derive the flow equations of all the physical parameters. Using
these results, we find that superconductivity can be strongly suppressed by the
ordering competition, and also extract the $T$-dependence of superfluid
density. Moreover, the phase transitions may become first order. Interestingly,
our RG analysis reveal that the nematic order exists only in an intermediate
temperature region $T_{m}< T < T_{n}$, but is destroyed at $T > T_{n}$ by
thermal fluctuation and at $T < T_{m}$ by ordering competition. This anomalous
existence of nematic order leads to a back-bending of the nematic transition
line on the phase diagram, consistent with the observed reentrance of
tetragonal structure at low temperatures. A modified phase diagram is obtained
based on the RG results.
