President Donald Trump signed executive orders Monday to beef up security at the southern border that began taking effect hours after he was inaugurated, making good on his defining political promise to crack down on immigration and marking another wild swing in White House policy on the divisive issue. Some of the orders revive priorities from his first administration that his predecessor had rolled back, including forcing asylum-seekers to wait in Mexico and finishing the border wall. Others created sweeping new strategies, like an effort to end automatic citizenship for anyone born in America, pulling the military into border security and ending use of a Biden-era app used by nearly a million migrants to enter America. Actual execution of such a far-reaching immigration agenda is certain to face legal and logistical challenges. And few details have been released so far. But in a concrete sign of how the changes were already playing out, migrants who had appointments to enter the U.S. using the CBP One app saw them canceled minutes after Trump was sworn in, and Mexico agreed to allow people seeking U.S. asylum to remain south of the American border while their court cases play out. “I will declare a national emergency at our southern border. All illegal entry will immediately be halted, and we will begin the process of returning millions and millions of criminal aliens back to the places in which they came,” Trump said in his inauguration speech to thunderous applause.