The dynamic evolution of a stretched magnetotail configuration is investigated numerically by means of a two-dimensional time-dependant nonlinear resistive MHD code. The initial configuration is shown to be stable in ideal MHD. The unstable evolution is initialted by imposing a finite resistivity. Rather independently of the boundary conditions, the following phenomena develop spontaneously: plasma sheet thinning, topological changes of the magnetic field configuration, the formation of neutral lines with an almost stationary X line, large induced cross-tail electric fields, conversion of magnetic to kinetic energy resulting in strong earthward and tailward flow close to the neutral sheet, and outside the neutral sheet a significant part of the flow parallel to the magnetic field. This flow reverses sign across the plasma sheet boundary tailward of the X line. The results support the tearing theory of magnetospheric substorms. At the later stages the configuration around the X line is similar to that obtained from steady state reconnection theory; in particular we found that two current layers form downstream of the magnetic field separatrix, which may represent structures developing into slow shock waves.