A Trump presidency doesn't necessarily mean the end of the United States as a policeman but it does mean that the US is going to be stepping back during the next four years from a number of roles that it's traditionally taken. The end of the Second World War marked the beginning of the era in which the United States had assumed the role of policing the world. But Donald Trump doesn’t think like that. He thinks more in transactional terms about alliances than any previous American president – certainly the only American president since the end of the Second World War.
I don't necessarily think that he's going to go through with all the promises that he made in the run up to the elections because I think a lot of what he said was campaign rhetoric.And in any case there will be a lot of other people in institutions like Congress, in the military and in the State Department whose main interests lie in maintaining things like American military bases abroad, upholding American alliances, keeping America’s membership prominent in NATO and that kind of thing. But certainly Donald Trump doesn't have the instinct to have the US carry out global policing duties in the interest of preserving the US-led world order, that presidents of both parties have done since the end of the Cold War.
"The problem is that the US has been the world's policeman for such a long time that we don't know what could happen with existing security arrangements."
But how this will play out is not clear. The problem is that the US has been the world's policeman for such a long time that we don't know what could happen with existing security arrangements. We just don’t know how they will hold with the US no longer as engaged.
Some things will no doubt require a strong police presence from the US. I think we can probably confidently assume that the major danger points are in the Middle East and in the Asia-Pacific region with Eastern Europe a possibility as well.
https://www.youtube.com/embed/rRV3H4AcM7Y?wmode=opaque
And here’s the worry: Trump will probably try to limit the US military deployments in Europe that are currently seen as a means of containing Russia. He will try to renegotiate the US relationship with Russia. But while the attitude towards Russia is expected to soften, by the same token he will probably be much more aggressive in the Pacific. Trump clearly thinks that the major strategic challenge to the United States is China, so he will be very friendly to Russia.