  Two-dimensional (2D) materials have become a fertile playground for the
exploration and manipulation of novel collective electronic states. Recent
experiments have unveiled a variety of robust 2D orders in highly-crystalline
materials ranging from magnetism to ferroelectricity and from superconductivity
to charge density wave (CDW) instability. The latter, in particular, appears in
diverse patterns even within the same family of materials with isoelectronic
species. Furthermore, how they evolve with dimensionality has so far remained
elusive. Here we propose a general framework that provides a unfied picture of
CDW ordering in the 2H polytype of four isoelectronic transition metal
dichalcogenides 2H-MX$_2$ (M=Nb, Ta and X=S, Se). We first show experimentally
that whilst NbSe$_2$ exhibits a strongly enhanced CDW order in the 2D limit,
the opposite trend exists for TaSe$_2$ and TaS$_2$, with CDW being entirely
absent in NbS$_2$ from its bulk to the monolayer. Such distinct behaviours are
then demonstrated to be the result of a subtle, yet profound, competition
between three factors: ionic charge transfer, electron-phonon coupling, and the
spreading extension of the electronic wave functions. Despite its simplicity,
our approach can, in essence, be applied to other quasi-2D materials to account
for their CDW response at different thicknesses, thereby shedding new light on
this intriguing quantum phenomenon and its underlying mechanisms.
