Description: The governing dynamics of numerically simulated cold fronts, as they collapse towards a minimum cross-frontal scale, are identified by determining force balances and the Rossby and Ekman numbers in frontal regions. A hierarchy of numerical simulations of cold fronts is performed with the Weather Research and Forecasting model and individual terms in the horizontal momentum equations are calculated from model output to construct force balances. An inviscid, three-dimensional, idealised front is simulated at three different horizontal grid spacings (100 km, 20 km and 4 km) and then the experiments are repeated with a planetary boundary layer (PBL) parameterization scheme included. Additionally, a full-physics simulation of the cold front originally analysed by Sanders (1955) is conducted. The leading edge of the idealised, inviscid cold front is characterised by small Rossby numbers and hence balanced dynamics at all three model resolutions. When the PBL scheme is included, large Rossby numbers and unbalanced flow develop at the leading edge of the cold front, but only when the front is simulated at high resolution; at coarse resolution, balanced dynamics remain. The acceleration is in the cross-front direction and arises due to a localised pressure gradient force. Unlike the inviscid experiments, or the coarse-resolution PBL experiments, the front in the high-resolution PBL experiment has a large cross-front thermal gradient, strong forced ascent, and a sharp surface pressure trough which enables the large cross-front pressure gradient force to develop. The dynamics of the Sanders (1955) front qualitatively agree with the idealised PBL simulation, but much larger Rossby numbers occur at the leading edge of the Sanders front. This finding indicates that the dynamics of the Sanders front are more unbalanced than the dynamics of the idealised simulations.