'If there was in fact a message, it was obviously "do not send Russian warplanes over the Turkish border"'
No, Putin got the message that was intended to be sent which is why he said that Russia was "stabbed in the back by accomplices of the terrorists". It is very likely that the message will be sent to him again.
'The price of oil is now less than half what it was in 2014'
Yes but Isis sell their oil to the smuggling gangs at well below the current market price and all the middlemen along the chain of tankers out of Syria and into Turkey make money which funds Isis and enables them to buy weapons and keep going.
'NATO forces have recently been attacking IS's oil extraction'
They stepped it up recently after Putin embarrassed them by hitting Isis oil.
Trump, the legend, said it all yet again
"Here’s what I would do. And I’ve been saying this for a long time, I have been saying it to you. I would have — and now they’re just starting — if you remember when I said, attack the oil, because that’s their primary source of wealth. Attack the oil. People smiled, and they laughed and they thought it was a joke, and they thought it was funny. Now as of two days ago, they’re attacking the oil."
Trump was giving this advice and they laughed at him.
"Obama Follows Trump Advice, Goes After Terrorists’ Oil
November 16, 2015
U.S. warplanes on Monday attacked trucks that ISIS is using to move oil, its chief source of revenue, in a move that GOP front runner Donald Trump has been pushing for months."
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