We have investigated the effect of an assembly history on the evolution of galactic dark matter (DM) halos of 1012 h-1 M☉ using constrained realizations of random Gaussian fields. Five different realizations of a DM halo with distinct merging histories were constructed and have been evolved using collisionless high-resolution N-body simulations. Our main results are as follows: A halo evolves via a sequence of quiescent phases of a slow mass accretion intermitted by violent episodes of major mergers. In the quiescent phases, the density is well fitted by an NFW profile, the inner scale radius Rs and the mass enclosed within it remain constant, and the virial radius (Rvir) grows linearly with the expansion parameter a. Within each quiescent phase the concentration parameter (c) scales as a, and the mass accretion history (Mvir) is well described by the Tasitsiomi et al. fitting formula. In the violent phases the halos are not in a virial dynamical equilibrium and both Rs and Rvir grow discontinuously. The violent episodes drive the halos from one NFW dynamical equilibrium to another. The final structure of a halo, including c, depends on the degree of violence of the major mergers and the number of violent events. Next, we find a distinct difference between the behavior of various NFW parameters taken as averages over an ensemble of halos and those of individual halos. Moreover, the simple scaling relations c-Mvir do not apply to the entire evolution of individual halos, and therefore we have the common notion that late-forming halos are less concentrated than early-forming ones. The entire evolution of the halo cannot be fitted by single analytical expressions.