The ability to rapidly learn about new environments and new spatial relationships is critical for survival. The hippocampus plays a crucial role in that learning, and specialized features of hippocampal population coding including network-level theta oscillatory activity, location-specific firing of principal cells, and reactivation of experience during immobility have been implicated in the rapid storage and retrieval of new spatial information. Disruptions of theta and reactivation jointly, or reactivation itself, are sufficient to impair learning, but the specific contribution of locomotion and theta-associated network activity remains unknown. In this study, we manipulated hippocampal activity by optogenetically stimulating septal parvalbumin-expressing GABAergic neurons in rats (6 transfected; 4 control). Our manipulations dynamically entrained hippocampal field potentials and neurons across a broad range of stimulation frequencies (58/58 epochs in transfected animals) while preserving the place code during stimulation times. We also developed a closed-loop, theta phase-specific stimulation protocol that was sufficient to reliably ablate theta power shortly after stimulation onset (20/20 epochs in transfected animals). Theta ablation was sufficient to impair learning in a spatial alternation task, producing pronounced deficits in the more cognitively challenging component of the task in transfected animals (4/4 animals tested). Similar stimulation patterns did not affect control animals (4/4 animals tested). Notably, network effects accompanying theta ablation were restricted to locomotion periods, and we did not observe changes in sharp-wave ripple rate or length during immobility. Our results are consistent with a critical role for locomotion-associated endogenous theta activity during the initial learning of a hippocampal-dependent task.