[en] Edge-Localized Modes (ELMs) is an intrinsic feature of the high confinement H-mode in tokamaks. On the one hand, they are beneficial for stationary discharge performance by expelling from the plasma impurities produced at the machine walls. On the other hand, under reactor conditions type I ELMs would themselves lead to a dramatic increase of impurity release and cause very large pulse heat loads on divertor plates. Therefore, it is crucial to find a way to mitigate ELMs without negative consequences for the plasma. Recent experiments on the tokamak D-III D have demonstrated that by applying resonant magnetic field perturbations from external coils, large type I ELMs can be effectively mitigated without any significant loss of confinement quality. This mode of operation is highly desirable for future fusion reactors and therefore it is very important to achieve an understanding of physical mechanism leading to ELMs suppression through external field perturbations. In the present contribution ballooning-pealing MHD instability considered normally as the main cause of the edge localized modes of type I, is theoretically examined in the presence of external magnetic field perturbations by applying a mode-mode coupling technique. It is demonstrated that through non-linear interaction, leading to the generation of side bands which suck energy from the main mode, such perturbations can raise the threshold of MHD instabilities. The synergy of this effect with the influence of external perturbations on the particle and energy transport in the edge transport barrier is studied by applying a simple model for the time variation of plasma parameters in the edge transport barrier. It is demonstrated that this synergy can be very beneficial for the plasma performance. In future experiments on JET and ITER it may lead to a noticeable increase of the pedestal pressure compared to the standard H-mode operation. (author)