  The ground state of the simple Heisenberg nearest-neighbor quantum kagome
antiferromagnetic model is a magnetically disordered spin liquid, yet various
perturbations may lead to fundamentally different states. Here we disclose the
origin of magnetic ordering in the structurally-perfect kagome material
YCu$_3$(OH)$_6$Cl$_3$, which is free of the widespread impurity problem. {\it
Ab-initio} calculations and modeling of its magnetic susceptibility reveal
that, similar to the archetypal case of herbertsmithite, the nearest-neighbor
exchange is by far the dominant isotropic interaction. Dzyaloshinskii-Moriya
(DM) magnetic anisotropy deduced from electron spin resonance and specific-heat
measurements is, however, significantly larger than in herbertsmithite. By
enhancing spin correlations within kagome planes, this anisotropy is essential
for magnetic ordering. Our study isolates the effect of DM anisotropy from
other perturbations and unambiguously confirms the theoretical phase diagram.
