Complex sentence: Semi-periphery nations are a necessary structural element in a world-trade system, since such nations can serve to alleviate the political pressures that the core can exert upon the periphery and the political unrest that the periphery can direct back at the core.
Split into simpler sentences: Semi-periphery nations are a necessary structural element in a world-trade system. In other words, the semi-periphery can serve to alleviate the political pressures that the core can exert upon the periphery and the political unrest that the periphery can direct back at the core.