ROMA, Texas – Stymied by Congress and the courts, President Donald Trump has struggled to make good on his signature campaign promises to build a wall and stop migrants. But there is at least one place where his vision is becoming reality: the sinuous lower Rio Grande Valley, scene of more unauthorized crossings than any other stretch between the Pacific Ocean and the Gulf of Mexico. This week, as Texas National Guard troops were taking up positions on the state's southern edge at Trump's request, the head of U.S.