Mr Trump was referring to his plan to pull US forces out of Syria, where they have been fighting alongside a Kurdish militia against the Islamic State (IS) group.
Turkey regards the Kurdish group as terrorists.
The US president’s comments drew a sharp response from Turkey, which said Mr Trump would not get anywhere by threatening the country economically.
After Mr Trump’s tweet, the Turkish lira dropped in value against the US dollar.
It then recovered its value, suggesting that his remarks had limited impact.
So how could the US damage Turkey economically – if it wanted to?
Close ties under strain
The relationship between Washington and Ankara has historically been close – politically, economically and militarily.
Turkey – a Nato member – is a vital partner for the US but there have been significant strains in the partnership.
These came out into the open last August, when the US slapped sanctions on Turkey over the continued detention of American pastor Andrew Brunson.
It marked a new low in relations and was a…