What is the Jones Act?

The Department of Homeland Security said Wednesday that it has not made a decision yet on whether to waive a nearly century-old shipping law that some say is hampering relief efforts in hurricane-ravaged Puerto Rico.

The Jones Act, otherwise known as the Merchant Marine Act of 1920, requires goods shipped between American ports to be carried out exclusively by ships built primarily in the United States, and to have U.S. citizens as its owners and crews.

Signed into law by President Woodrow Wilson two years after World War I ended, the Jones Act was passed as a protective measure against foreign competition, particularly Germany. By restricting domestic trade to U.S.-flagged vessels with U.S. crews, America would always have a robust fleet of boats and sailors on hand in the event German submarines attacked the U.S. Read more at NBCNews.com.

Check out these 9 essential things to know about Puerto Rico's humanitarian crisis shared by Vox.com.