  We show that space-time resonances induce high-frequency Raman instabilities
in the two-fluid Euler-Maxwell system describing laser-plasma interactions. A
consequence is that the Zakharov WKB approximation to Euler-Maxwell is unstable
for non-zero group velocities. A key step in the proof is the reformulation of
the set of resonant frequencies as the locus of weak hyperbolicity for
linearized equations around the WKB solution. We analyze those linearized
equations with the symbolic flow method. Due to large transverse variations in
the WKB profile, the equation satisfied by the symbolic flow around resonant
frequencies is a linear {\it partial} differential equation. At space-time
resonances corresponding to Raman instabilities, we observe a fast growth of
the symbolic flow, which translates into an instability result for the original
system. The strongest instability is caused by backscattered Raman waves.
