Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) US troops have deployed to Bulgaria as part of a NATO operation to support Eastern European allies from potential aggression by Russia, the Bulgarian Ministry of Defense said in a statement. The move is the latest in a series of multinational training and security cooperation activities along the alliance's eastern front. It comes at a tenuous time in the relationship between the Kremlin and the fledgling administration of President Donald Trump, who just last week voiced unusually strong support for NATO after repeatedly objecting to its relevancy and funding. One hundred twenty troops from the 3rd Armored Brigade Combat Team, 4th Infantry Division, from Fort Carson, Colorado, arrived Wednesday at the Novo Selo training range in eastern Bulgaria, according to the Bulgarian statement. They're due to be joined by heavy military equipment and armored vehicles by the end of the week.More than 100 service members from Colorado now part of missionDeployment comes as Trump, often critical of NATO, affirms US commitment 
 Question:It is with this aim that _ continues its expansion," Putin said.

NATO