The latest Homeland Security Department data show that about 702,000 visitors who entered the country by sea or air in fiscal 2017 remained longer than allowed, according to a Government Accountability Office review. By comparison, U.S. authorities apprehended 304,000 migrants at the southern border that year.
The Homeland Security data showed a handful of African countries had among the highest proportion of visa overstays, especially Eritrea, Chad, Liberia and Somalia.
The Republic of Djibouti, a tiny but strategically vital country on the Horn of Africa, had a suspected overstay rate of 41.6%, the highest proportion of any nation. Djibouti has a population of about 1 million people.
Nigeria, which has about 200 million people, had an overstay rate of 10.3%, but 19,046 overstays, far more than Djibouti.
Last year, Trump made disparaging remarks about African countries, expressing a preference for immigration from northern European countries like Norway.
Brown, who worked for the George W. Bush and Barack Obama administrations, said it made sense for Trump to focus on countries with a high proportion of overstays but a relatively small number in total.
"If we were to start talking about sanctions and visa denials to a country like Canada or Mexico, there would be a whole lot more national interests to consider than some of these countries in Africa," she said. "It'd be harder to do."
Trump's memo says if the countries with high overstay rates don't reduce them, the United States may toughen visa requirements or shorten visits.
Under current policy, countries with overstay rates of at least 2% are required to initiate public awareness campaigns warning against staying beyond permission, according to the White House.
Even as it focused on curbing overstays, the White House said the goal was to alleviate the "significant strain" overstays put on the Justice and Homeland Security departments, saying their resources "are currently needed to address the national emergency on our southern border."