A relativistic paradox seemingly violating conservation of momentum law in electromagnetic systems

Two identical electric point charges that move away from each other are considered. To an observer at rest in a reference frame in which the charges move with equal velocities, the forces exerted by the charges upon each other appear equal in magnitude and opposite in direction, so that there is no net force on the system of the two charges. However, to an observer at rest in a reference frame in which one of the charges is stationary, the two forces appear unequal, so that the charges appear to generate a net force upon themselves. The resolution of this paradox is a very instructive exercise in relativity and electromagnetism.