Complex sentence: Flynn served as the 25th National Security Advisor from January 20,2017, to February 13,2017, when he resigned at the behest of the President after misleading key officials about the nature of his telecommunications with Russian diplomats.
Split into simpler sentences: Flynn served as the twenty-fifth national security advisor from January 20,2017, to February 13,2017. He resigned at the behest of the president after information surfaced regarding his misleading key officials about the nature of his telecommunications with Russian diplomats.