President Donald Trump’s sweeping new travel ban came into effect early Monday immediately after midnight, barring citizens from a dozen nations from entering the United States and reviving a divisive measure from his first term.

The move is expected to disrupt refugee pathways and further restrict immigration as the Trump administration expands its crackdown on illegal entries.

Many of the nations covered by the restrictions have adversarial relations with the United States, such as Iran and Afghanistan, while others face severe crises, like Haiti and Libya.

In announcing his restrictions last week, Trump said the new measure was spurred by a recent “terrorist attack” on Jews in Colorado.

The group had been protesting in solidarity with hostages held in Gaza when they were assaulted by a man the White House said had overstayed his visa.

That attack, Trump said, “underscored the extreme dangers posed to our country by the entry of foreign nationals who are not properly vetted” or who overstay their visas.

The move bans all travel to the United States by nationals of Afghanistan, Myanmar, Chad, Congo-Brazzaville, Equatorial Guinea, Eritrea, Haiti, Iran, Libya, Somalia, Sudan and Yemen, according to the White House.