We've been trying that for decades. More than 1000 miles of border - if you just consider Texas to California, and ignore also Pacific Ocean or Gulf of Mexico routes. Easier said than done.
I'll admit this administration cannot even seem to secure the fence around the White house, so it may be beyond their ability, assuming they even wanted to slow down the flow of illegals. However, it is clearly feasible if we were serious about doing it. We could remove most of the incentive if he denied illegals any benefits amd required secure verification of status for employment.
Emails: UN health agency resisted declaring Ebola emergency http://www.wral.com/emails-un-health-agency-resisted-declaring-ebola-emergency/14526731/ By early June of last year, the Ebola epidemic in West Africa was the deadliest ever recorded. There weren't enough beds for patients and many were refusing to seek treatment, driving the outbreak underground. Senior staffers in Africa at the World Health Organization raised the prospect of declaring an international emergency. The answer from WHO's Geneva headquarters: Wait. According to internal emails and documents obtained by The Associated Press, the U.N. health agency resisted sounding the international alarm until August, a two-month delay that some argue may have cost lives. More than 10,000 are believed to have been killed by the virus since WHO announced the outbreak a year ago. WHO has acknowledged acting too slowly to control the Ebola epidemic. In its defense, the agency says the virus' spread was unprecedented and blames factors including lack of resources and intelligence from the field. Internal documents obtained by AP, however, show WHO's top leaders were informed of how dire the situation was. But they held off on declaring an emergency in part because it could have angered the countries involved, interfered with their mining interests or restricted the Muslim pilgrimage to Mecca in October. (More at above url)