Complex sentence: Meanwhile in Katanga, after the UN Secretary-General, Dag Hammarskjöld, was killed in an airplane crash, the UN began to take a more aggressive stance towards the secessionists.
Split into simpler sentences: Meanwhile in Katanga, after the UN Secretary-General, Dag Hammarskjöld, was killed in an airplane crash. His successors in the UN took a more aggressive stance towards the secessionists.