President Donald Trump has threatened to close the southern U.S. border between if Mexico does not implement a plan to stop illegal immigration into the United States.

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Trump tweeted on Friday that he would shut down “the Border, or large sections of the Border,” if Mexico does not “immediately stop ALL illegal immigration coming into the United States throug (sic) our Southern Border.”

Trump told reporters in Florida last week, "If they (Mexico) don't stop them, we are closing the border. We'll close it. And we'll keep it closed for a long time. I'm not playing games."

While Trump has threatened such action in the past, and for a brief time closed the border crossing at San Ysidro near San Diego, he has not ordered any part of the country's southern bordered closed.

What would happen if he does decide to close the border? Here's a look at trade between the two countries and what would happen if the border closes.

What would closing border do? 

Mexico is the United States’ third-largest trading partner, behind China and Canada, so closing a border can affect many things.

Prices would rise on items such as produce and auto parts, should the flow of products to the United States be cut off.

In addition to goods that come across the border, jobs are also in danger.

The Department of Commerce estimates that U.S. exports of goods and services to Mexico support upwards of 1.2 million jobs.

What goes through border?

According to the Office of the United States Trade Representative, the U.S. traded $615.9 billion in goods and services with Mexico last year. The U.S. imported $339.8 billion and exported $276.2 billion.

According to the U.S. Department of Transportation, $206 billion of those goods were shipped to Mexico via truck or rail.

Some of the goods the U.S. exports to Mexico include steel mill products, agricultural machinery, repair parts for cars and trucks, aircraft and aircraft parts and oil production equipment.

Mexico, in return, supplies 44 percent of the imported produce to the United States, including tomatoes, cucumbers and raspberries.

Lance Jungmeyer, president of the Fresh Produce Association of the Americas, told The Washington Post that while the winter produce season is winding down, watermelon, avocados and table grapes would be affected by a border shutdown now.

>> US avocado supply will dry up in 3 weeks if Trump shuts border with Mexico, produce company says

According to the Laredo, Texas, Chamber of Commerce, the Port of Laredo handled 50 percent of the U.S.-Mexico goods traded in 2017. That port saw an average of 16,000 trucks and 1,400 rail cars cross the city’s border each day.

According to the U.S. Chamber of Commerce, "So many area residents cross the border daily to work, shop or attend school that the San Ysidro, California, Chamber of Commerce estimates that the local economy suffered lost revenues of $5.3 million just in the five-hour period the border was closed last November."

How would it affect state economies?

Mexico is Texas' largest trade partner. They traded $18.7 billion in goods and services in 2018.

California traded $6 billion with Mexico, while Michigan traded $5.3 billion and Illinois $2 billion.

If border is shut down, does that literally mean no one can drive back, forth? What about vacation travel?

If closing the border means truly closing down the border to car, train or foot traffic, then it would affect vacation travel or travel to visit family, or friends in Mexico if travelers get there by car or train.

Would a shutdown include air travel? 

That’s not known yet.

If the shutdown stops trade, how will the stock market respond?

What goes on with America’s third-largest trading partner would likely have an effect on the stock market, which could affect many areas of investing, including 401K accounts.

What are Northern Triangle countries administration is talking about? What do they have to do with migrant caravans?

Trump directed the State Department on Friday to revoke aid from the three countries in Central America referred to as the Northern Triangle countries. Trump said those countries do not do enough to stop migrant caravans as they head north toward the southern border of the U.S.

The countries -- Guatemala, El Salvador and Honduras -- received millions of dollars a year in U.S. aid. The caravans that head north through Mexico largely originate in those countries.

When could shutdown happen?

Trump said the shutdown could happen as early as this week if Mexico does not step up its efforts to stem the tide of immigrants.

According to the Post, neither Border Protection officials, military officials nor Congress has been notified as of yet that there may be a border closing.

Would border shutdown stop people from entering country? 

A border shutdown would not necessarily stop people from entering the country across the southern border.

A person must only step on American soil to request asylum in the United States, and whether the border is officially open or closed, there are many areas where undocumented immigrants can physically enter the country.

What is asylum; how does process work?

When someone is requesting asylum in the United States, they are asking for protection from a credible threat of harm against them in their native country. The threat can be because of their race, religion, nationality or membership in a particular social or political group. 

The U.S. government is bound by United Nations treaties and U.S. law to protect persecuted people who show up at the country’s border. Anyone seeking asylum in the U.S. must be granted an interview to determine if they are eligible to be considered for asylum.

To apply for asylum, immigrants must be physically present in the United States, meaning they must step foot on American soil.

According to U.S. law, "Any alien who is physically present in the United States, or who arrives in the United States (whether or not at a designated port of arrival and including an alien who is brought to the United States after having been interdicted in international or United States waters), irrespective of such alien's status, may apply for asylum."

Asylum seekers must generally apply for the protection within one year of their arrival to the United States. 

How is that different from being refugee who wants to come to United States?

A person who requests protection in the United States while still outside U.S. borders, and is then given permission to enter the United States is called a refugee.

“Refugee status is a form of protection that may be granted to people who meet the definition of refugee and who are of special humanitarian concern to the United States.

Refugees are generally people outside of their country who are unable or unwilling to return home because they fear serious harm," according to U.S. Citizenship and Immigration Services.

Under the Immigration and Nationality Act, there is a cap on the number of refugees who may enter the United States in a given year. That number comes from the president's recommendation to Congress. 

Both those seeking asylum and those who receive refugee status must prove they qualify for protection under U.S. law.

Cars and trucks line up to enter Mexico from the U.S. at a border crossing in El Paso, Texas, Friday, March 29, 2019. Threatening drastic action against Mexico, President Donald Trump declared on Friday he is likely to shut down America's southern border next week unless Mexican authorities immediately halt all illegal immigration. Such a severe move could hit the economies of both countries, but the president emphasized, "I am not kidding around." 
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