In a March 2000 BBC Interview, when asked if Russia could join NATO, Putin then-Acting President, said:

"Why not? I don't rule out such a possibility".

Later the same year, Putin apparently raised the idea with then-President Bill Clinton, saying something to the extent, "Let's consider an option that Russia might join NATO." And Clinton replied, "Why not".

To make this accession talk even more serious, Putin raised the point with then-NATO Secretary General George Robertson. According to Robertson, Putin even insisted that Russia should be invited to join NATO as he felt Russia was too important for having to stand in line with other countries, waiting for a possible accession.

Eventually Putin was told that this was not the way it worked, that a country wanting to join needed to file a formal application.

As we all know, Russia did not join NATO. As some say, Putin felt "snubbed" having to apply as other "minor countries". He wanted to be treated as an "equal" partner, whatever that meant. Maybe, he felt Russia should be treated as "more equal" than others.

Well, that did not work out. But not just for this minor reason. The Kremin and of course President Putin himself started realizing that NATO was expanding ever more eastwards, despite the promises made in 1990 by the Allies, when German Unification was discussed:

"NATO will move not one inch east of Berlin", said then-US Secretary of State James Baker. He said it in February 1990 to then-Soviet leader Mikhail Gorbachev.

Though not in writing, such a political verbal commitment has legal standing.

Early in the first decade of the current millennium, President Putin began seeing NATO's continued eastward expansion – despite the 1990 promise – as an increasing threat to Russia's security. The Russian Security forces, from which Putin then and now, receives a lot of support, viewed already then membership in a western alliance as a betrayal of Russia.