Mr Trump has acknowledged that his ban on travel from 26 European countries will have a “big impact”.
Let's put some numbers to that.
Last March, almost a third of America’s overseas visitors - about 850,000 people - came from European countries affected by the ban. They spent about $3.4bn, the US Travel Association estimates.
The ban will cancel more than 7,300 flights from Europe to the US, or more than 2 million seats, with Germany and France accounting for the largest numbers, according to analysis by Cirium.
No wonder airline shares were slammed on Thursday and countries such as France were offering government assistance.
Delta Air Lines and United – which Cirium said have the most affected flights - both saw shares drop more than 14%.
Airlines were bracing for a more than $100bn hit even before this ban. But the ramifications extend well beyond the industry.
Travel drives more than 10% of the global economy, accounting for 1 in 10 jobs worldwide, according to the World Travel & Tourism Council. In recent years it has been among the fastest growing sectors.
So if it’s hurting, others will too.