Studying the Progenitors of Our Favorite Clusters at z>1

Surveys in the 2020s, notably the CMBS4 program and Athena, will reveal galaxyclusters at high redshifts, all the way back to zvirgul3, when they first formed.Unveiling the properties (density, temperature, entropy, metallicity) andevolution of the intracluster medium and the galaxyhalo connection in theseearlyforming systems will be among the primary science goals of both Athena andany Chandra successor. While many of the most exciting questions about theinitial formation of galaxy clusters must wait for nextgeneration Xraymissions, we can lay an important foundation now by extending studies ofstatistical cluster samples into the z>1 regime. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]