  We present deep near-infrared images, taken with Subaru Telescope, of the
region around the z=1.08 radio source 3C 356 which show it to be associated
with a poor cluster of galaxies. We discuss evidence that this cluster
comprises two subclusters traced by the two galaxies previously proposed as
identifications for 3C 356, which both seem to harbour AGN, and which have the
disturbed morphologies expected if they underwent an interpenetrating collision
at the time the radio jets were triggered. We explain the high luminosity and
temperature of the diffuse X-ray emission from this system as the result of
shock-heating of intracluster gas by the merger of two galaxy groups. Taken
together with the results on other well-studied powerful radio sources, we
suggest that the key ingredient for triggering a powerful radio source, at
least at epochs corresponding to z~1, is a galaxy--galaxy interaction which can
be orchestrated by the merger of their parent subclusters. This provides an
explanation for the rapid decline in the number density of powerful radio
sources since z~1. We argue that attempts to use distant radio-selected
clusters to trace the formation and evolution of the general cluster population
must address ways in which X-ray properties can be influenced by the radio
source, both directly, by mechanisms such as inverse-Compton scattering, and
indirectly, by the fact that the radio source may be preferentially triggered
at a specific time during the formation of the cluster.
