Complex sentence: When he returned to Ottawa, the Riel crisis was in full swing on what to do with Louis Riel, who had been sentenced to hang for leading the 1885 North-West Rebellion, which was now the responsibility of Thompson.
Split into simpler sentences: When he returned to Ottawa, the Riel crisis was in full swing. The question of what to do with Louis Riel, who had been sentenced to hang for leading the 1885 North-West Rebellion, was now the responsibility of Thompson.