Kurt Volker, the State Department's special envoy for Ukraine, resigned Friday amid a formal impeachment inquiry of President Trump and his communications with the Ukrainian government, including the country's president, Volodymyr Zelensky. Volker did not provide a public explanation for leaving his post, but a source familiar with his decision said Volker concluded he could not perform the job effectively as a result of the recent developments.One person familiar with the matter told NBC News that Volker's resignation will likely enable him to be much freer in what he can say about his time at his post if he is called at some point to testify before Congress.The whistleblower complaint that sparked the impeachment inquiry alleges that Volker went to Kiev to help guide Ukrainian officials on how to handle Trump's alleged demands that the government investigate former Vice President Joe Biden's son, Hunter. He also reportedly spoke with Trump's personal lawyer Ru...