  Starting from a microscopic itinerant model, we derive and analyze the
effective low-energy model for collective magnetic excitations in the iron
pnictides. We show that the stripe magnetic order is generally preempted by an
Ising-nematic order which breaks $C_{4}$ lattice symmetry but preserves O(3)
spin-rotational symmetry. This leads to a rich phase diagram as function of
doping, pressure, and elastic moduli, displaying split magnetic and nematic
tri-critical points. The nematic transition may instantly bring the system to
the verge of a magnetic transition, or it may occur first, being followed by a
magnetic transition at a lower temperature. In the latter case, the preemptive
nematic transition is accompanied by either a jump or a rapid increase of the
magnetic correlation length, triggering a pseudogap behavior associated with
magnetic precursors. Furthermore, due to the distinct orbital character of each
Fermi pocket, the nematic transition also induces orbital order. We compare our
results to various experiments, showing that they correctly address the changes
in the character of the magneto-structural transition across the phase diagrams
of different compounds, as well as the relationship between the orthorhombic
and magnetic order parameters.
