Chad Irby wrote:

You also have some problems with ionization of the air in some
conditions, degrading the beam.

For visible or near IR, this is not so much of a problem as with mid IR
(or UV, for that matter). There is an ionization phenomena that can
actually help propagate the laser beam in some conditions. Very intense
light in air can lead to self focusing, and if there was nothing to stop
it, the laser would catastrophically self focus down to a point,
resulting in strong ionization and the total absorption of the beam. It
turns out however that before this occurs, the beam will cause weak
ionization of the air, forming a diverging lense and expanding the beam
again. The beam still has enough power to self focus in normal air,
though, so you go through a sort of leap-frog effect of focus, diverge,
focus, diverge, etc. This overcomes diffractive spreading of the beam,
and some researchers have managed to propagate millijoule, femptosecond
pulses of laser light for several kilometers through the atmosphere
using this method. It is not clear if this would be a good option for
weapons, but it might turn out to be a very effective means of
delivering pulsed laser energy to targets within a few kilometers.

Luke