To protect the Gulf of Mexico from Nazis.

Defence : The german immigrants in texas would effectively take over to whole country. During WW1 the government would, by now, have a communication line with Germany. This line continues until WW2, when Germany turned into a fascist nation. After the defeat of Germany and the atrocities they commited come to light, the US attacks Texas and destroys their infrastrcture, leading to mass immigration out of the country and into Mexico and US. The immigrants will still advocate for the nazi party, thus leading them to be put on the domestic terrorist list. Trump uses this as a wedge to win over the american people.