  Young massive clusters (YMCs) are usually accompanied by lower-mass clusters
and unbound stars with a total mass equal to several tens times the mass of the
YMC. If this was also true when globular clusters (GCs) formed, then their
cosmic density implies that most star formation before redshift ~2 made a GC
that lasted until today. Star-forming regions had to change after this time for
the modern universe to be making very few YMCs. Here we consider the conditions
needed for the formation of a ~10^6 Msun cluster. These include a star
formation rate inside each independent region that exceeds ~1 Msun/yr to sample
the cluster mass function up to such a high mass, and a star formation rate per
unit area of Sigma_SFR ~ 1 Msun/kpc^2/yr to get the required high gas surface
density from the Kennicutt-Schmidt relation, and therefore the required high
pressure from the weight of the gas. High pressures are implied by the virial
theorem at cluster densities. The ratio of these two quantities gives the area
of a GC-forming region, ~1 kpc^2, and the young stellar mass converted to a
cloud mass gives the typical gas surface density of 500-1000 Msun/pc^2
Observations of star-forming clumps in young galaxies are consistent with these
numbers, suggesting they formed today's GCs. Observations of the cluster
cut-off mass in local galaxies agree with the maximum mass calculated from
Sigma_SFR. Metal-poor stellar populations in local dwarf irregular galaxies
confirm the dominant role of GC formation in building their young disks.
