1. He signalled and called for the border wall with Mexico before meeting with the Mexican President, rather than working the details out first. After all, this is a shared border.
2. Then he aid Mexico would pay for the wall, without any agreement on the other side or discussion.
3. Then Mexico said we wont pay for the wall and cancelled the meeting
4. So now they want a tariff on Mexican imports as a knee jerk reaction and Mexico will retaliate in some fashion.
Mexico is the 15th largest economy in the world, the second largest export market to the United States and third largest import market. Their most significant exports? Parts used by the US auto industry, medical devices used by the healthcare industry, and food - particularly produce. 68% of US produce in the winter months comes from Mexico.
Way to fuck up a whole bunch of trade over a nothing issue. Now, US cars will cost more, food will cost more, and most diagnostic tests will cost more. And we got nothing out of it.
The "greatest negotiator in the world" just gave us a giant L.
Edit: another significant export for Mexico: Concrete. Which you need for roads, buildings, and other structures. And the stupid wall.