It was made during a verbal question and answer session some days ago. You can read a transcript of the full thing, without western media's blatantly selective quoting and bias, right here [kremlin.ru]. Do go read it for yourself. The press has been having a field day with taking individual sentences out of context, in many cases not even mentioning that Putin was responding to questions from Russian citizens, to make it look like he's issuing press releases about Ukraine specifically. It's the most amazingly dangerous set of selective quotations I've ever seen. In this case Putin wasn't even talking about Ukraine!

LAKE SELIGER, Russia — President Vladimir Putin said Friday that Russia's armed forces, backed by its nuclear arsenal, were ready to meet any aggression, declaring at a pro-Kremlin youth camp that foreign states should understand: "It's best not to mess with us."

these would have ended up transitioning over the next few decades to either other European countries or to the US as having Russian bases in a NATO country wouldn't be acceptable to NATO or Russia.

Not forced into a corner exactly, but there are a lot of people who regard themselves as Russian living there and who he is expected to defend. Their government, whatever you think of it, was overthrown and they wanted to go back to Russian rule. Particularly in Crimea they were Russian citizens until not that long ago, and all of it was under Soviet rule until the late 80s.

There is massive propaganda on both sides, but what can't be doubted is that Russia is making serious efforts to get a political dialogue going between the separatists and Ukrainian government.

Russia was at risk of losing access to a vital military facility

LAKE SELIGER, Russia — President Vladimir Putin said Friday that Russia's armed forces, backed by its nuclear arsenal, were ready to meet any aggression, declaring at a pro-Kremlin youth camp that foreign states should understand: "It's best not to mess with us."

Russian GRU defector Stanislav Lunev said in his autobiography that "the GRU and the KGB helped to fund just about every antiwar movement and organization in America and abroad," and that during the Vietnam War the USSR gave $1 billion to American anti-war movements, more than it gave to the VietCong.[19] Lunev described this as a "hugely successful campaign and well worth the cost".[19] According to Time magazine, a US State Department official estimated that the KGB may have spent $600 million on the peace offensive up to 1983, channeling funds through national Communist parties or the World Peace Council "to a host of new antiwar organizations that would, in many cases, reject the financial help if they knew the source."[13] Richard Felix Staar in his book Foreign Policies of the Soviet Union says that non-communist peace movements without overt ties to the USSR were "virtually controlled" by it. Lord Chalfont claimed that the Soviet Union was giving the European peace movement £100 million a year. The Federation of Conservative Students (FCS) alleged Soviet funding of CND.

In 1985 Time magazine noted "the suspicions of some Western scientists that the nuclear winter hypothesis was promoted by Moscow to give antinuclear groups in the U.S. and Europe some fresh ammunition against America's arms buildup."[20] Sergei Tretyakov claimed that the data behind the nuclear winter scenario was faked by the KGB and spread in the west as part of a campaign against Pershing missiles.[21] He said that the first peer-reviewed paper in the development of the nuclear winter hypothesis, "Twilight at Noon" by Paul Crutzen and John Birks (1982),[22] was published as a result of this KGB influence.

I hope very much that not just Russia’s historical memory but that all of humanity will prompt us to search for peaceful solutions to the various conflicts that are currently unfolding and that will arise in the future. We support political dialogue and the search for compromise.