  Despite the progress made in successful prediction of many classes of
weakly-correlated topological materials, it is not clear how a topological
order can emerge from interacting orders and whether or not a charge ordered
topological state can exist in a two-dimensional (2D) material. Here, through
first-principles modeling and analysis, we identify a 2$\times$2 charge density
wave (CDW) phase in monolayer $2H$-NbSe$_2$ that harbors coexisting quantum
spin Hall (QSH) insulator, topological crystalline insulator (TCI) and
topological nodal line (TNL) semimetal states. The topology in monolayer
NbSe$_2$ is driven by the formation of the CDW and the associated
symmetry-breaking periodic lattice distortions and not via a pre-existing
topology. Our finding of an emergent triple-topological state in monolayer
$2H$-NbSe$_2$ will offer novel possibilities for exploring connections between
different topologies and a unique materials platform for controllable
CDW-induced topological states for potential applications in quantum
electronics and spintronics and Majorana-based quantum computing.
