  We develop a first-principles model for the relativistic magnetic
reconnection rate in strongly magnetized pair plasmas. By considering the
energy budget and required current density near the x-line, we analytically
show that in the magnetically-dominated relativistic regime, the x-line thermal
pressure is significantly lower than the upstream magnetic pressure due to the
extreme energy needed to sustain the current density, consistent with kinetic
simulations. This causes the upstream magnetic field lines to collapse in,
producing the open outflow geometry which enables fast reconnection. The result
is important for understanding a wide range of extreme astrophysical
environments, where fast reconnection has been evoked to explain observations
such as transient flares and nonthermal particle signatures.
