  URu$_2$Si$_2$ is surely one of the most mysterious of the heavy-fermion
compounds. Despite more than twenty years of experimental and theoretical
works, the order parameter of the transition at $T_0 = 17.5$ K is still
unknown. The state below $T_0$ remains called "hidden-order phase" and the
stakes are still to identify the energy scales driving the system to this
phase. We present new magnetoresistivity and magnetization measurements
performed on very-high-quality single crystals in pulsed magnetic fields up to
60 T. We show that the transition to the hidden-order state in URu$_2$Si$_2$ is
initially driven by a high-temperature crossover at around 40-50 K, which is a
fingerprint of inter-site electronic correlations. In a magnetic field
$\mathbf{H}$ applied along the easy-axis $\bf{c}$, the vanishing of this
high-temperature scale precedes the polarization of the magnetic moments, as
well as it drives the destabilization of the hidden-order phase. Strongly
impurity-dependent magnetoresistivity confirms that the Fermi surface is
reconstructed below $T_0$ and is strongly modified in a high magnetic field
applied along $\mathbf{c}$, i.e. at a sufficiently-high magnetic polarization.
The possibility of a sharp crossover in the hidden-order state controlled by a
field-induced change of the Fermi surface is pointed out.
