  We present the results of a Chandra X-ray survey of the 8 most massive galaxy
clusters at z>1.2 in the South Pole Telescope 2500 deg^2 survey. We combine
this sample with previously-published Chandra observations of 49 massive
X-ray-selected clusters at 0<z<0.1 and 90 SZ-selected clusters at 0.25<z<1.2 to
constrain the evolution of the intracluster medium (ICM) over the past ~10 Gyr.
We find that the bulk of the ICM has evolved self similarly over the full
redshift range probed here, with the ICM density at r>0.2R500 scaling like
E(z)^2. In the centers of clusters (r<0.1R500), we find significant deviations
from self similarity (n_e ~ E(z)^{0.1+/-0.5}), consistent with no redshift
dependence. When we isolate clusters with over-dense cores (i.e., cool cores),
we find that the average over-density profile has not evolved with redshift --
that is, cool cores have not changed in size, density, or total mass over the
past ~9-10 Gyr. We show that the evolving "cuspiness" of clusters in the X-ray,
reported by several previous studies, can be understood in the context of a
cool core with fixed properties embedded in a self similarly-evolving cluster.
We find no measurable evolution in the X-ray morphology of massive clusters,
seemingly in tension with the rapidly-rising (with redshift) rate of major
mergers predicted by cosmological simulations. We show that these two results
can be brought into agreement if we assume that the relaxation time after a
merger is proportional to the crossing time, since the latter is proportional
to H(z)^(-1).
