President Donald Trump’s executive order barring citizens of 12 countries from entering the United States officially went into effect Sunday night.

The presidential proclamation, issued last week, cites the need to “protect the United States from foreign terrorists and other national security threats.” The ban affects citizens of Afghanistan, Burma (Myanmar), Chad, Republic of the Congo (Congo-Brazzaville), Equatorial Guinea, Eritrea, Haiti, Iran, Libya, Somalia, Sudan, and Yemen.

The Trump administration, known for its strict immigration policies, justified the move by pointing to the lack of effective governmental control in these countries, which hampers proper vetting of travelers. Additionally, U.S. officials noted a tendency among citizens of some of the listed countries to overstay their visas.