In network systems, a local perturbation can amplify as it propagates, potentially leading to a largescale cascading failure. Here we derive a continuous model to advance our understanding of cascading failures in power-grid networks. The model accounts for both the failure of transmission lines and the desynchronization of power generators, and incorporates the transient dynamics between successive steps of the cascade. In this framework, we show that a cascade event is a phase-space transition from an equilibrium state with high energy to an equilibrium state with lower energy, which can be suitably described in closed form using a global Hamiltonian-like function. From this function we show that a perturbed system cannot always reach the equilibrium state predicted by quasi-steadystate cascade models, which would correspond to a reduced number of failures, and may instead undergo a larger cascade. We also show that in the presence of two or more perturbations, the outcome depends strongly on the order and timing of the individual perturbations. These results offer new insights into the current understanding of cascading dynamics, with potential implications for control interventions.Cascading processes underlie a myriad of network phenomena [1], including blackouts in power systems [2,3], secondary extinctions in ecosystems [4,5], and complex contagion in financial networks [6,7]. In all such cases, an otherwise small perturbation may propagate and eventually cause a sizable portion of the system to fail. Various system-independent cascade models have been proposed [8][9][10][11][12][13] and used to draw general conclusions, such as on the impact of interdependencies [14] and countermeasures [15]. There are outstanding questions, however, for which it is necessary to model the cascade dynamics starting from the actual dynamical state of the system.In power-grid networks, the state of the system is determined by the power flow over transmission lines and the frequency of the power generators, which must be respectively below capacity and synchronized under normal steady-state conditions. Although a local perturbation has a limited impact on the connectivity of the network, it may trigger a cascade of failures and protective responses that switch off grid components and may also lead generators to lose synchrony. Much of our current understanding about this process has been derived from quasi-steady-state cascade models [16][17][18][19][20][21], which use iterative procedures to model the successive inactivation of network components caused by power flow redistributions, while omitting the transient dynamics between steady states as well as the dynamics of the generators. Further understanding has resulted from stability studies focused on the synchronization dynamics of power generators in the absence of flow redistributions [22][23][24][25][26].Yet, to date no theoretical approach has been developed to incorporate at the same time these two fundamental aspects of power-grid dynamics-frequency change and flow ...