  The physics of star formation and the deposition of mass, momentum, and
energy into the interstellar medium by massive stars (`feedback') are the main
uncertainties in modern cosmological simulations of galaxy formation and
evolution. These processes determine the properties of galaxies, but are poorly
understood on the $\lesssim$100 pc scale of individual giant molecular clouds
(GMCs) resolved in modern galaxy formation simulations. The key question is why
the timescale for depleting molecular gas through star formation in galaxies
($t_{\rm dep}\approx 2$ Gyr) exceeds the dynamical timescale of GMCs by two
orders of magnitude. Either most of a GMC's mass is converted into stars over
many dynamical times, or only a small fraction turns into stars before the GMC
is dispersed on a dynamical timescale. Here we report our observation that
molecular gas and star formation are spatially de-correlated on GMC scales in
the nearby flocculent spiral galaxy NGC300, contrary to their tight correlation
on galactic scales. We demonstrate that this de-correlation implies rapid
evolutionary cycling between GMCs, star formation, and feedback. We apply a
novel statistical method to quantify the evolutionary timeline and find that
star formation is regulated by efficient stellar feedback, driving GMC
dispersal on short timescales (~1.5 Myr) due to radiation and stellar winds,
prior to supernova explosions. This feedback limits GMC lifetimes to about one
dynamical timescale (~10 Myr), with integrated star formation efficiencies of
only 2-3%. Our findings reveal that galaxies consist of building blocks
undergoing vigorous, feedback-driven lifecycles, that vary with the galactic
environment and collectively define how galaxies form stars. Systematic
applications of this multi-scale analysis to large galaxy samples will provide
key input for a predictive, bottom-up theory of galaxy formation and evolution.
