Complex sentence: Trygve Lie, who after the war had become the first Secretary-General of the United Nations, characterized Koht in his memoirs, noting that Koht was an expert on foreign affairs, but that he was introvert.
Split into simpler sentences: Trygve Lie, who after the war had become the first Secretary-General of the United Nations, characterized Koht in his memoirs. Lie noted that Koht was an expert on foreign affairs, but that he was introvert.