Mr Trump says the US is pulling out of the Intermediate Range Nuclear Forces Agreement. Now that dates back to the end of the Cold war. 1987, Reagan and Gorbachev met and agreed that neither country would have land based ground-launched missiles of intermediate range from 500km to 5,500km.
And it was really all about keeping Europe safe from the prospect of nuclear war. At the time, it marked the first real rallentando in the Cold war. It was the first time that either side dismantled any of its nuclear arsenal, and the numbers actually reduced.
The US says that Russia has been in violation of this treaty since 2014. And Mr Trump thinks that the US has now been waiting for quite long enough. And Russia, for its part, alleges that the US is also in violation of it. It thinks its missile defence facilities in Europe could be turned into missile launchers. And so they don't feel safe.
For the US part, the claim is that some of the Russian missiles that currently exist are within that intermediate range. And Nato increasingly has seemed to agree with the US administration that Russia is in violation of the treaty. And given that the US has been flagging this since 2014, they think that it's been simply too long, waiting for Russia to fall back into compliance with both sides slinging mud at each other.
Mr Trump is obviously keen on his hard lines. He's announced withdrawal, but he hasn't actually given a date. And there's a little bit of sense there's still some wiggle room on this. But that, ultimately, the US will likely withdraw.
But perhaps the most interesting read on what Trump might be up to is to look at China. Now, China is not part of this essentially US-Russia treaty. So China has been able to build intermediate range missiles throughout this period.
And, indeed, a senior US defence official has previously said that 95 per cent of all of China's missiles are within this intermediate range. And one of the reasons that Russia has been annoyed about this treaty in the past is because it has not been able to develop intermediate range missiles directly to counter what it might see as the Chinese threat.
And so neither the US nor the Russians have been able to counter, measure up to what the Chinese have been producing in really quite significant numbers. And when Trump said that he was withdrawing the US from this treaty, he specifically mentioned China. In a sort of caveat way, maybe he could make a deal, might China like to come into the treaty.
Now, there are not many people, who I've spoken to, who think it's at all likely that China would voluntarily join this treaty. It would have to destroy a huge number of missiles that it has decided are good for its defence. But it does show that what the US is really worried about long-term is less the threat of Russia and more the threat of China.