  The emergence of unconventional superconductivity in proximity to intertwined
electronic orders is especially relevant in the case of iron-based
superconductors. Such order consists of an electronic nematic order and a spin
density wave in these systems. BaNi$_2$As$_2$, like its well-known iron-based
analog BaFe$_2$As$_2$, also hosts a symmetry-breaking structural transition
that is coupled to a unidirectional charge density wave (CDW), providing a
novel platform to study intertwined orders. Here, through a systematic
angle-resolved photoemission spectroscopy study combined with a detwinning
$B_1g$ uniaxial strain, we identify distinct spectral evidence of band
evolution due to the structural transition as well as CDW-induced band folding.
In contrast to the nematicity and spin density wave in BaFe$_2$As$_2$, the
structural and CDW order parameters in BaNi$_2$As$_2$ are observed to be
strongly coupled and do not separate in the presence of uniaxial strain. Our
measurements point to a likely lattice origin of the CDW in BaNi$_2$As$_2$.
