Setting the stage for a NATO summit dominated by the worst East-West tensions in decades, President Barack Obama will go to Estonia this week with a simple message for Vladimir Putin -- don't mess with NATO's ex-Soviet members.
With Europe and the United States accusing the Russian president of sending his troops to intervene directly in Ukraine, Obama's trip to Europe, his second of the year, comes at a dangerous moment.
Obama will make clear that NATO views its Article Five creed on common defense as inviolate, and his first visit as president to the Baltic states -- long seen as a potential flashpoint of any Western military clash with Russia -- is highly symbolic.
The only other sitting president to visit Latvia, Lithuania or Estonia, the trio of former Soviet republics, was President George W Bush, in 2006, when calm prevailed across former Cold War frontiers.
"There is a perception in Eastern Europe and in the Baltics that Putin poses not just a threat to Ukraine, but he does actually pose a long-term threat to NATO, because his long-term strategic goal in this view is to undermine the US alliance system in Europe, or to show that it's hollow," said Thomas Wright, of the Brookings Institution.
"The best way of doing that is to show that Article Five is hollow; that it doesn't mean what it says it means," said Wright, referring to the NATO pledge that binds member nations to view an attack on one country as an attack on the entire membership.