"If You Weren't Worried Before, Worry Now"
The US needs to stop playing serious hardball with opponents who have for years been screwing the US in various ways.
"Even if Mexico simply closes its own borders, north and south, this will leave a sour taste in the mouth for years on all sides."
Dear douchenozzle, the reason Trump is acting to stop illegal immigration the US has had a bad taste in its mouth after decades of massive illegal immigration. Mexico loves illegal immigration since it acts as a safety valve for the incompetent Mexican government and provides huge inflows of US dollars. The US generally tolerates illegal immigration, but even we have a limit.
The limit has been reached, either Mexico will work to address the problem, or it will suffer for its intransigence.