We propose a model that is valid for ultrafast pulse propagation in a mode-locked laser cavity in the few-femtosecond pulse regime, thus deriving the equivalent of the master mode-locking equation for ultrashort pulses that has dominated mode-locking theory for two decades. The short-pulse equation with dissipative gain and loss terms allows for the generation of stable ultrashort optical pulses from initial white noise, thus providing the first theoretical framework for quantifying the pulse dynamics and stability as pulses widths approach the attosecond regime.