Active-duty military personnel began arriving along the southern border Thursday evening after President Donald Trump declared a national emergency to address the immigration crisis.

Active duty troops in Texas will be building permanent and temporary physical barriers to stop illegal border crossings, using intelligence analysts for the monitor and detection of migrants and military aircrafts to take detainees back to their country of origin.

Troops have assisted at the border before and will be limited to support roles unless the administration invokes the Insurrection Act.