  We study the large-time behavior of Eulerian systems augmented with non-local
alignment. Such systems arise as hydrodynamic descriptions of agent-based
models for self-organized dynamics, e.g., Cucker-Smale and Motsch-Tadmor models
\cite{CS,MT}. We prove that in analogy with the agent-based models, the
presence of non-local alignment enforces \emph{strong} solutions to
self-organize into a macroscopic flock. This then raises the question of
existence of such strong solutions. We address this question in one- and
two-dimensional setups, proving global regularity for \emph{sub-critical}
initial data. Indeed, we show that there exist \emph{critical thresholds} in
the phase space of initial configuration which dictate the global regularity
vs. a finite time blow-up. In particular, we explore the regularity of nonlocal
alignment in the presence of vacuum.
