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tokens: that minihalos that have been chemically enriched by supernova ejecta from prior Population III star formation can cool and form stars and, moreover, significantly contribute to the overall ionizing photon budget of reionization. We refer to this new class of halos as metal-line cooling halos, or MCs. Follow-on simulations (the Renaissance Simulations) in much larger volumes by Xu et al. ( 2016b ) provide the star formation rates (SFRs), intermittency, and ionizing escape fractions in the MCs, LMACHs, and HMACHs with extremely high resolution and good statistics. Using these results as our input, we revisit the problem: what is the role of the lowest-mass halos in the reionization process? The main improvement of our work compared with previous work is that the simulations shown here are fully coupled cosmological radiation hydrodynamic simulations, with a time-dependent treatment of the ionization kinetics and emissivities assigned to the source halos dynamically, considering the intermittency of the contribution from MCs. We find that because star formation is intermittent in MCs, the earliest phases of reionization exhibit a stochastic nature, with small H ii regions forming and recombining. Only later, after the characteristic halo mass scale has reached ∼10 9 , does reionization proceed smoothly in the usual manner deduced from earlier studies. Adopting concordance cosmological parameters and only using the galaxy properties from the Renaissance Simulations, our 1152 3 simulation in a 14.4 comoving Mpc box begins reionizing at z = 20, is 10% ionized at z = 10, and fully ionizes at z = 7.1. This paper is organized as follows. We summarize the relevant results from Xu et al. ( 2016b ) in Section 2 . The description of computational method and inputs to the simulations is provided in Section 3 . We show results in Section 4 and offer a discussion and conclusions in Section 5 . 2. Ionizing Photons from the Smallest Galaxies Unlike the previous papers in this series (So et al. 2014 ; Norman et al. 2015 ), where ionizing emissivities were calculated from a simple star formation/feedback recipe incorporated in the simulation itself, here we employ results from much higher resolution simulations that calculate the escaping ionizing photons of high-redshift galaxies directly. How this is done is described in Sections 3.2 and 3.3 . This approach enormously relaxes the spatial resolution requirement on the global reionization simulation and takes advantage of more precise simulation results. Uniform grids may be employed for the reionization simulation; however, they must have sufficient mass and spatial resolution to accurately capture the halo population of importance. In addition, we are able to use moment methods for the radiation transport, which do not get bogged down as reionization completes as some ray-tracing methods do (Norman et al. 2015 ). We draw on the results of Xu et al. ( 2016b ), who performed three high-resolution AMR