The primary reason I'd nuke the Ross Ice shelf is because it would speed up the process of flooding the planet, sweeping the lowlands of continents and filling them with recently re-introduced glacial meltwater. Effectively controlling the population of the planet. Since marine mammals and a few scientists (whom I can assure you don't stay there year round) inhabit the vast expanse of the ross ice shelf, which is essentially the size of France. By targeting near the break point, you'd have some serious melting action going on down there.
Also, it wouldn't be an attack on just one nation, but an attack on all the lowlands of the planet. The purpose is simply to control the world's population.