Narrowing the discussion to only Mexican nationals, a 2015 study performed by demographers of the University of Texas at San Antonio and the University of New Hampshire found that immigration from Mexico; both legal and illegal, peaked in 2003 and that from the period between to the period of 2008 to .Immigrants from countries that do not have automatic visa agreements, or who would not otherwise qualify for a visa, often cross the borders illegally in some areas like the United States-Mexico border, the Mona Channel between the Dominican Republic and Puerto Rico, the Strait of Gi.ltar, Fuerteventura, and the Strait . - America will create a huge decrease in the number of illegal aliens by building this wall and keeping Mexicans in Mexico, and there will be a big jump in American If you want to identify and reduce illegal immigration, you need to focus on identifying likely overstays and making sure they return. The wall . - There were 11.7 million immigrants from Mexico living in the U.S. in 2014, and about half of them were in the country illegally, according to Pew Research Center estimates. Mexico is the country 's largest source of immigrants, making up 28 of all U.S. immigrants. With President Donald Trump 's .