President Trump on Wednesday signed two executive orders–one to immediately begin to construct a border wall with Mexico and one to deny federal funds to so-called sanctuary cities that shield undocumented immigrants from deportation.

“A nation without borders is not a nation,” Trump said Wednesday at the Department of Homeland Security, where he signed the orders alongside the newly sworn-in Secretary John F. Kelly.  “Beginning today, the United States of America gets back control of its borders,” Trump said

President Trump has repeatedly said that Mexico will ultimately pay for the wall, but Mexican officials have maintained that they have no intention of doing so.  Mexican President Enrique Pena Nieto and President Trump had a scheduled business meeting next Tuesday to discuss immigration, trade, and drug-war cooperation.  However, a decision was made to scrap the meeting in the wake of  tense disagreement of who would pay for the border wall between the United States and Mexico.  President Pena Nieto said that calling for Mexico to foot the cost of the wall is an affront to his country’s sovereignty and dignity, but his government is was willing to work with the United States “to reach agreements that benefit both nations.”

Trump spokesman Sean Spicer said yesterday that President Trump intended to pay for the wall by imposing a 20-percent tax on all imports from Mexico. Spicer was not clear exactly how the Trump administration would impose the new tax on Mexican exports but said it would be part of a broader plan to tax imports from countries with which the United States has a trade deficit, such as Mexico.  He also said that the White House would “look for a date to schedule the U.S-Mexican meeting in the future.” Spicer’s announcement sent the peso tumbling.

Mexico is the second largest customer for American-made products in the world, and 80 percent of Mexican exports–automobiles, flat-screen TVs, avocados–are sold to the United States.

Congress would have to appropriate new funding for the construction of the wall, which some have estimated could cost tens of billions of dollars.  Republican leaders said today that they will move forward with plans to approve funds for a wall they estimate would cost $12-$15 billion.  President Trump has directed federal agencies to use existing funds as a start to the wall and formally called for the hiring of an additional 5,000 Border Patrol agents and 10,000 immigration officers.

The 2,000-mile border wall could take years to plan, fund, and build.  The wall would also face serious geographic and legal constraints along the way.  Much of the border between the United States and Mexico, particularly in Texas, runs along private property, through state and national parks, and through areas with natural barriers that already limit illegal crossings.

There are an estimated 11 million undocumented immigrants in the United States.  Trump’s executive order on immigration revives programs that allow the federal government to work with local and state-law enforcement agencies to arrest and detain unauthorized immigrants with criminal records and to share information to help track and deport them.  However, critics say the executive order language is so broad that it could include anyone in the country without legal status.

President Trump is expected to sign another executive order today that bans for 30 days visa entries from seven countries including Syria, Iraq, Iran, Yemen, Somalia, Sudan and Libya. Those who already have a visa will not be allowed to enter.