  Recently, several putative quantum spin liquid (QSL) states were discovered
in ${\tilde S} = 1/2$ rare-earth based triangular-lattice antiferromagnets
(TLAF) with the delafossite structure. A way to clarify the origin of the QSL
state in these systems is to identify ways to tune them from the putative QSL
state towards long-range magnetic order. Here, we introduce the Ce-based TLAF
KCeS$_2$ and show via low-temperature specific heat and $\mu$SR investigations
that it yields magnetic order below $T_{\mathrm N} = 0.38$ K despite the same
delafossite structure. We identify a well separated ${\tilde S} = 1/2$ ground
state for KCeS$_2$ from inelastic neutron scattering and embedded-cluster
quantum chemical calculations. Magnetization and electron spin resonance
measurements on single crystals indicate a strong easy-plane $g$~factor
anisotropy, in agreement with the ab initio calculations. Finally, our
specific-heat studies reveal an in-plane anisotropy of the magnetic
field-temperature phase diagram which may indicate anisotropic magnetic
interactions in KCeS$_2$.
