Community assembly theory investigates the mechanisms through which species from a broader pool of potential colonizers form local communities at finer spatiotemporal scales. The theory is heuristic because it reduces the large number of possible mechanisms shaping communities into a tractable number of fundamental high-level processes. However, despite its heuristic value, community assembly theory is inherently context-dependent, i.e., its predictions regarding community dynamics are only valid within specific ecological conditions. Thus, a synthetic understanding of community assembly relies on identifying a few influential ecological axes that regulate the context-dependent nature of community dynamics.  In this thesis, I set out to investigate community assembly along two latent ecological axes that determine the context of community dynamics. The first represents the top-down control of species pools on the membership of local communities. The second represents the bottom-up control of landscape features on species movement and interactions. By employing process-based simulation models that replicated community assembly across varied landscape structures and species pool compositions, I generated theoretical predictions about the isolated and interactive effects of both forms of control on: (i) spatiotemporal patterns in community composition; (ii) the ecological selection of prevailing life-history strategies observed in (meta)communities; (iii) the relative importance of assembly processes across space and time and throughout large-scale ecological gradients; and (iv) the trajectories of communities (towards differentiation or homogenization) in response to natural or anthropogenic disturbances. I provide empirical validation for these theoretical predictions by investigating the assembly of insect communities in distinct (bio)geographic contexts or by contrasting model predictions with empirical patterns observed in the literature. In parallel, I introduced new analytical frameworks that allowed the testing of the predictions outlined in this thesis and can be used to address other pertinent questions in community ecology. Collectively, the chapters in this thesis derive a mechanistic understanding of causal links between landscape-mediated bottom-up control, species pool-mediated top-down control, and the context-dependent nature of community assembly. Beyond its theoretical significance, this knowledge is crucial for predicting how the impact of human activities on landscapes and species pools can alter the structure, dynamics, and regulation of ecological communities.