(This happens whenever a government promises to spend a lot on infrastructure improvements.)
2. Foreign investors anticipate this and are buying US dollars. This makes the dollar stronger w.r.t. other currencies.
I think a strong currency is usually a good thing in a scarce market environment. It means that your country can buy a lot of the world's natural resources easier than others. This should usually be a good thing (for the US).
But there is a problem:
The US doesn't actually do anything with natural resources anymore, we don't make things like steel or plastic etc anymore, or rather, we don't "add value" to natural resources. Instead, with a strong dollar, we let others add the value and (import) buy their finished goods. The few things that we do make become more expensive to others, so our exports decrease. Domestically, there are fewer "manufacturing" jobs.
Trump will try to stop this and the only tool he has is to increase import tariffs. This article seems to equate that with global financial ruin, and it is the point of contention that I want to raise.
These two facts: raising import tariffs and a strong US dollar may help cancel out the effects. Instead of destroying foreign markets, it may simply "transfer" who gets the surplus generated by the strong dollar from the foreign markets to our tax base.