Explainer: How does the situation on the U.S.-Mexico border compare with the past?

FILE PHOTO: Migrants are seen after crossing illegally to the United States to turn themselves in to request asylum in El Paso, Texas, U.S., as seen from Ciudad Juarez, Mexico May 31, 2019. REUTERS/Jose Luis Gonzalez/File Photo

(Reuters) - U.S. President Donald Trump has made immigration a central theme of his administration but has grown increasingly frustrated with the ballooning numbers of mostly Central American families crossing the U.S.-Mexico border and turning themselves in to U.S. authorities.

A threat of across-the-board tariffs on Mexican goods starting on Monday unless Mexico does more to stop migrants from reaching the United States has led to high-stakes negotiations between the two governments.