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.

"If he can prove that in one instance, he basically discredits Article Five for NATO for as a whole, and he would discredit NATO as a whole."

- Vulnerable again -

Obama will arrive in Tallin in the early hours of Wednesday and meet Estonian President Toomas Hendrik Ilves, Lithuanian President Dalia Grybauskaite and Lavtia's president Andris Berzins.

The three republics, which joined NATO in 2004, spent five decades under Soviet rule, and feel themselves vulnerable to the kind of covert warfare pioneered by Moscow in Ukraine owing to their substantial Russian minorities.

Obama previewed his firm message to the Kremlin in Washington last week.

"Part of the reason I'll be going to Estonia is to let the Estonians know that we mean what we say with respect to our treaty obligations," Obama said, drawing a distinction between the Baltics and non-NATO member Ukraine.

Charles Kupchan, Obama's top White House official for European affairs, was even more blunt about a message Obama will also carry to the NATO summit in Wales on Thursday and Friday.

"Russia, don't even think about messing around in Estonia or in any of the Baltic areas in the same way that you have been messing around in Ukraine."

Though the president warned Ukraine, as a non-NATO state did not enjoy the same security guarantees as the Baltic states, he announced on Thursday that he would meet Ukrainian President Petro Poroshenko at the White House on September 18.

Obama came to office determined to "reset" ties with the Kremlin but Putin's return as president scuppered the initiative and questions of territory and defense, thought resolved when the Cold War ended, are back on the agenda.

Edward Lucas, an author and researcher with the Center for European Policy Analysis in Washington, says Obama is now sending the kind of signals familiar to his White House predecessors during the age of the Iron Curtain.

"This is an extremely bleak moment in European security, far bleaker than many people realize," said Lucas, adding that Obama must make a statement that will resonate in history, akin to John Kennedy's "Ich bin ein Berliner" speech in 1963.

"Obama has tried every possible style with Putin. He has tried being nice, he has tried ignoring him, he has tried being tough," Lucas said.

"Obama is under no illusion what Putin is like. But the truth is, Obama is busy and Europe is not one of his top priorities," he said, at a time when the US leader is also consumed by the rise of Islamic State in Iraq and Syria.

- The West, weak in resolve? -

One consideration worrying close observers of US-Russia relations is whether Putin, who appears to believe the West is weak in resolve, divided and unfocused, truly understands US intent.

Asked on Friday whether Russia understood that interfering in the Baltics was intolerable for the United States, White House spokesman Josh Earnest said that given Moscow's "irrational" explanations for its conduct in Ukraine, no one could be sure.

That perception seems to be shared in the Baltic states, once again, threatening to become a front line between the West and the East.

"Russia is at war against Ukraine and that is against a country which wants to be part of Europe. Russia is practically in war against Europe," said Grybauskaite, in Brussels, for a European Union summit.