Last week also brought reports that Moscow is increasing its troop presence in Crimea and along its borders with Ukraine.

Joe Biden responded. In his first conversation with Ukrainian President Volodymyr Zelensky, Biden assured him of our "unwavering support for Ukraine's sovereignty and territorial integrity in the face of Russia's ongoing aggression in the Donbass and Crimea."

Though Ukraine is not a member of NATO, and we have no treaty obligation to fight in its defense, this comes close to a war guarantee. Biden seems to be saying that if it comes to a shooting war between Moscow and Kiev, we will be there on the side of Kiev.

Last week, Kremlin spokesman Dmitry Peskov answered that if the U.S. sends troops to Ukraine, Russia will respond.