Thousands of active-duty US troops are immediately deployed to the US-Mexico frontier to tighten security and provide back up to overstretched border agents.
US authorities say troops are needed to address large groups of migrants arriving at the border in so-called caravans, made up largely of Central American migrants.
The issue has incensed President Donald Trump, who has sought to make political capital by depicting the migrants as carrying out an “invasion” of America.
He has repeatedly hammered on the caravans and tried to keep the issue in the headlines ahead of crucial midterm congressional elections that could see the Democrats regain some degree of power.
The Democrats, for their part, have seemed to founder in terms of presenting a coherent message on border and immigration issues and the caravans.