Illinois joined a list of states filing a lawsuit challenging President Donald Trump’s new executive order aiming to end birthright citizenship.

Illinois Attorney General Kwame Raoul announced the lawsuit Tuesday, arguing Trump’s order violates the constitutional rights to which all children born in the U.S. are entitled.

“That one of Donald Trump’s first day[s] in office as president should be so diametrically opposed to our values as Americans is incredibly disappointing, though not surprising. The children born in the U.S. to immigrants are entitled to the rights and privileges that go along with U.S. citizenship,” Raoul said in a statement. “We need to discuss bipartisan commonsense immigration reforms, but denying birthright citizenship, which dates back centuries and has been upheld twice by the U.S. Supreme Court, is not the solution. As Attorney General, and as the proud son of Haitian immigrants, I will continue to stand with my fellow attorneys general to defend the constitutional rights of all children born in this country.”

Attorneys general from more than a dozen other states also sued to block Trump’s move.

The order would end a decades-old immigration policy known as birthright citizenship guaranteeing that U.S.-born children are citizens regardless of their parents’ status.

Trump’s roughly 700-word executive order, issued late Monday, amounts to a fulfillment of something he’s talked about during the presidential campaign. But whether it succeeds is far from certain amid what is likely to be a lengthy legal battle over the president’s immigration policies.

Here’s a closer look at birthright citizenship, Trump’s executive order and reaction to it:

What is birthright citizenship?

Birthright citizenship means anyone born in the U.S. is a citizen, regardless of their parents’ immigration status. People, for instance, in the United States on a tourist or other visa or in the country illegally can become the parents of a citizen if their child is born here.

It’s been in place for decades and enshrined in the 14th Amendment to the Constitution, supporters say. But Trump and allies dispute the reading of the amendment and say there need to be tougher standards on becoming a citizen.

What does Trump’s order say?

The order questions that the 14th Amendment extends citizenship automatically to anyone born in the United States.

The 14th Amendment was born in the aftermath of the Civil War and ratified in 1868. It says: “All persons born or naturalized in the United States and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.”

Trump’s order excludes the following people from automatic citizenship: those whose mothers were not legally in the United States and whose fathers were not U.S. citizens or lawful permanent residents; people whose mothers were in the country legally but on a temporary basis and whose fathers were not citizens or legal permanent residents.

It goes on to bar federal agencies from recognizing the citizenship of people in those categories. It takes effect 30 days from Tuesday, on Feb. 19.

What is the history of the issue?

The 14th Amendment did not always guarantee birthright citizenship to all U.S.-born people. Congress did not authorize citizenship for all Native Americans born in the United States, for instance, until 1924.

In 1898 an important birthright citizenship case unfolded in the U.S. Supreme Court. The court held that Wong Kim Ark, who was born in San Francisco to Chinese immigrants, was a U.S. citizen because he was born in the country. After a trip abroad, he had faced denied reentry by the federal government on the grounds that he wasn’t a citizen under the Chinese Exclusion Act.

But some advocates of immigration restrictions have argued that while the case clearly applied to children born to parents who are both legal immigrants, it’s less clear whether it applies to children born to parents without legal status.

What has the reaction to Trump’s order been?

More than a dozen states, plus the District of Columbia and San Francisco, sued in federal court to block Trump’s order.

Raoul and multiple other states said the executive order means, for the first time since the 14th Amendment was adopted in 1868, that babies born each month who would have been citizens will no longer enjoy the privileges and benefits of citizenship.

“If allowed to stand, the infants stripped of their United States citizenship under the executive order will lose their most basic rights and will be forced to live under the threat of deportation. They will lose eligibility for a wide range of federal benefits programs, including their ability to obtain a Social Security number and, as they age, to work lawfully. They will also lose their rights to vote, serve on juries, and to secure passports. Despite the Constitution’s guarantee of citizenship, thousands of children will – for the first time – lose their ability to fully and fairly participate in American society as a citizen,” Raoul said in a statement.

Illinois Gov. J.B. Pritzker said the state plans to “fight back.”

“Here in Illinois we are going to stand up, we are going to fight back. We are going to follow the law and we are going to make sure they are following the law,” he said.

New Jersey Democratic Attorney General Matt Platkin said Tuesday that presidents might have broad authority but they are not kings.

“The president cannot, with a stroke of a pen, write the 14th Amendment out of existence, period,” he said.

Connecticut Attorney General William Tong, a U.S. citizen by birthright and the nation’s first Chinese American elected attorney general, said the lawsuit was personal for him.

“The 14th Amendment says what it means, and it means what it says —- if you are born on American soil, you are an American. Period. Full stop,” he said. “There is no legitimate legal debate on this question. But the fact that Trump is dead wrong will not prevent him from inflicting serious harm right now on American families like my own.”

Not long after Trump signed the order, immigrant rights groups filed suit to stop it.

Chapters of the American Civil Liberties Union in New Hampshire, Maine and Massachusetts along with other immigrant rights advocates filed a suit in New Hampshire federal court.

The suit asks the court to find the order to be unconstitutional. It highlights the case of a woman identified as “Carmen,” who is pregnant but is not a citizen. The lawsuit says she has lived in the United States for more than 15 years and has a pending visa application that could lead to permanent status. She has no other immigration status, and the father of her expected child has no immigration status either, the suit says.

“Stripping children of the ‘priceless treasure’ of citizenship is a grave injury,” the suit said. “It denies them the full membership in U.S. society to which they are entitled.”

Other states that have filed suit include California, Massachusetts, Colorado, Connecticut, Delaware, Hawaii, Maine, Maryland, Michigan, Minnesota, Nevada, New Mexico, New York, North Carolina, Rhode Island, Vermont, and Wisconsin.