Planet of the Damned (SF Classic). Harry Harrison

Brion Brandd, lives on the planet Anvhar, which, due to an elliptical orbit, experiences a year with a long cold winter and a short hot summer, to which the population have become adapted. To avoid social problems during the winter period, Anvhar has initiated a planet-wide series of mental and physical games called the Twenties. The novel starts with Brandd winning the Twenties. As he recovers from the games, Brandd meets Ihjel, a previous winner of the Twenties, who asks him to join a mission on the desert planet of Dis. The ruling class of Dis, the magter, have threatened to transport cobalt bombs onto a neighbouring planet if they refuse to surrender. As a result, the planet is being blockaded and under threat of a pre-emptive nuclear strike.