Complex sentence: He tried to return to Slovakia, but was stopped by Hungarian border guards, who turned him over to the Slovakian authorities, who in turn sent him to the Nováky transition camp, a holding camp for Jews awaiting deportation.
Split into simpler sentences: He tried to return to Slovakia, but was stopped by Hungarian border guards, who handed him to the Slovakian authorities. They in turn sent him to the Nováky transition camp, a holding camp for Jews awaiting deportation.