 -- The acting U.S. Attorney General, Sally Yates, was fired Monday night hours after she refused to defend President Trump's executive order regarding immigration because she is not convinced it is "lawful," according to a letter.

In a statement, the White House said Yates "betrayed the Department of Justice" and was "weak on borders" and said she was being replaced by Dana Boente, the U.S. Attorney for the Eastern District of Virginia.

Boente moved quickly to rescind the guidance in Yates' letter, noting in a statement released Monday night that the department's Office of Legal Counsel "found the Executive Order both lawful on its face and properly drafted." Boente then directed staffers "to do our sworn duty and to defend the lawful orders of our President."

"Ms. Yates is an Obama Administration appointee who is weak on borders and very weak on illegal immigration," the White House statement added. "It is time to get serious about protecting our country. Calling for tougher vetting for individuals travelling from seven dangerous places is not extreme. It is reasonable and necessary to protect our country."

In the letter, Yates wrote: "At present, I am not convinced that the defense of the Executive Order is consistent with these responsibilities nor am I convinced that the Executive Order is lawful."

"Consequently, for as long as I am the Acting Attorney General, the Department of Justice will not present arguments in defense of the Executive Order, unless and until I become convinced that it is appropriate to do so."

Trump's order, which went into effect Friday, affects immigration from seven countries that have predominantly Muslim populations, temporarily bans certain travelers and places indefinite restrictions on Syrian refugees.

The order sparked protests at airports across the country, as well as legal action. A federal judge issued a stay on part of the order regarding deportations.

Yates, who was nominated by former President Obama and confirmed in May 2015, wrote that the "order has now been challenged in a number of jurisdictions."

"My responsibility is to ensure that the position of the Department of Justice is not only legally defensible, but is informed by our best view of what the law is after consideration of all the facts," she wrote. "In addition, I am responsible for ensuring that the positions we take in court remain consistent with this institution’s solemn obligation to always seek justice and stand for what is right."

She also wrote that the Office of Legal Counsel's review of the order "does not address whether any policy choice embodied in an Executive Order is wise or just."

Before Trump signed the executive order, DOJ's Office of Legal Counsel offered the White House a "narrow" assessment of whether the orders were illegal on their face, according to a Justice Department official.

"Through administrations of both parties, the Office of Legal Counsel (OLC) has consistently been asked by the White House to review Executive Orders for form and legality before they are issued. That review is limited to the narrow question of whether, in OLC's view, a proposed Executive Order is on its face lawful and properly drafted. OLC has continued to serve this traditional role in the present administration," the DOJ official said a statement.

The office's legal review is generally conducted without the involvement of DOJ leadership, and OLC's legal review "does not address the broader policy issues inherent in any executive order," the statement continued.

According to a DHS official, as of Sunday night, there were 735 encounters at ports of entry related to the executive order and no one remains detained.

In addition, there was confusion over whether or not the executive order applied to green card holders from the countries, until DHS Sec. John Kelly issued a statement Sunday, ordering that the entry of lawful permanent residents was in "the national interest," allowing those people to enter the U.S. absent any "significant derogatory information indicating a serious threat to public safety and welfare."

After Yates' letter was released, Trump Tweeted on Monday night that the "Democrats are delaying my cabinet picks for purely political reasons. They have nothing going but to obstruct. Now have an Obama A.G."

And on MSNBC, White House adviser Stephen Miller said that the acting attorney general's memo is a "demonstration of how politicized our legal system has become" and defended the president’s authority to determine who is allowed to enter the country.

White House Press Secretary Sean Spicer defended the implementation of the president’s immigration executive order during his press briefing Monday, telling ABC News' Cecilia Vega that the interests of national security outweighed the uncertainty that was caused to those who were stuck in limbo when they arrived in the U.S.

"Coming into this country is a privilege, and be able to come to America is a privilege not a right and it is our duty and it is the president's goal to make sure that everyone who comes to this country to the best of our ability is here because they want to enjoy this country and come in peacefully and so he takes that obligation extremely seriously," he said.

Jack Date and Jordyn Phelps contributed reporting to this story.