As Russian President Vladimir Putin's forces took over Ukraine's Crimean Peninsula in early 2014, the interim Ukrainian government was debating whether or not to fight back against the "little green men" Russia had deployed. But the message from the Barack Obama administration was clear: Avoid military confrontation with Moscow.

The White House's message to Kiev was advice, not an order, U.S. and Ukrainian officials have recently told us, and was based on a variety of factors. There was a lack of clarity about what Russia was really doing on the ground. The Ukrainian military was in no shape to confront the Russian Spetsnaz (special operations) forces that were swarming on the Crimean Peninsula. Moreover, the Ukrainian government in Kiev was only an interim administration until the country would vote in elections a few months later. Ukrainian officials told us that other European governments sent Kiev a similar message.

But the main concern was Russian President Vladimir Putin.