Why are strong foreign relations important to the US and Texas?

It is one of the largest contributors to both countries' economies:

• For the US, it represents 8% of the total population and 7.95% of its GDP (ranked second in both terms after California).
• For Mexico, it represents by far the largest business and trade partner: Texas accounts for 23% and 21% of total exports, only followed by California (9% of total exports) and China (15% of total imports)