If China hadn’t provided warning enough to Donald Trump by seizing an American vessel in the South China Sea and threatening sanctions on an unnamed US automaker, president Obama had clear advice at a press conference today: think things through before you mess with Taiwan.

It’s not clear how much foreign policy issues have come up in Obama and Trump’s regular phone calls, but the president’s clearly spelled-out message seems to imply his successor is yet to fully grasp China’s vision of itself, or of the way negotiations play out with a global superpower. The quote is worth reading in full:

“Understand that with China, the issue of Taiwan is as important as anything on their docket. The idea of ‘one China’ is at the heart of their conception as a nation. And so if you are going to upend this understanding, you have to have thought through what the consequences are, because the Chinese will not treat that the way they will treat some other issues. This goes to the core of how they see themselves, and their reaction on this issue could end up being very significant. That doesn’t mean you have to adhere to everything that’s been done in the past…it does mean that you have to think it through.”

If you look at that in context with a statement from the same press conference, you can get an idea of how little Trump might have gamed out his tactics on China: “Since there’s only one president a time, my advice to him is that, before he starts having a lot of interaction with foreign governments, beyond the usual courtesy calls, he should have his full team in place,” Obama said.

The overall message is: it’s your foreign policy but make damn sure you know what steps two, three, four and down to twenty are before going straight for the motherload from the word “go.” Because this isn’t Manhattan real estate, it’s an ancient country with a strong sense of identity and a lot of tools at its disposal, beginning with the kinds of cyber-attacks it’s been prying away with for years.

Instead, the way Trump’s acted so far has been the real world equivalent of his reported advice to a US official in 1990 on how to strike a “great deal” with the Soviets on nuclear arms: “Trump told him to arrive late, stand over the Soviet negotiator, stick his finger in his chest, and say, ‘Fuck you!,'” the New Yorker‘s Evan Osnos reported earlier this year.

Beyond angering China and unleashing potentially serious consequences for the US, Obama also subtly pointed out that using Taiwan as a negotiating chip could end up very badly for the island itself. The current uneasy balance of Taiwanese relative autonomy without self-determination, he argued, “has kept the peace and given the Taiwanese an economy of people who have some degree of self-determination.”

His point? That the wrong moves could quickly see Taiwan’s peace and self-determination go the sad way of the heavily-oppressed Uighurs in China’s restive north-western region of Xinjiang. That’s not what anyone needs.