A renewed push by President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE to resume a policy that resulted in the separation of migrant children from their families at the U.S. border with Mexico led to the resignation Sunday of Homeland Security Secretary Kirstjen Nielsen Kirstjen Michele NielsenDHS IG won't investigate after watchdog said Wolf, Cuccinelli appointments violated law Appeals court sides with Trump over drawdown of immigrant protections Democrats smell blood with new DHS whistleblower complaint MORE, according to a report from NBC News.

Nielsen had resisted the directive, according to NBC, which cited three unidentified U.S. officials with knowledge of meetings at the White House. It said Trump for months has pressured officials to resume the separations.

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Nielsen’s resistance strained her relationship with Trump, NBC reported, adding that the outgoing secretary told the president she could not reinstate the policy due to federal court orders and an executive order he signed last year ending it.

Trump reportedly first began urging the return of the policy in January amid a spike in undocumented immigrants crossing the border and is convinced separating families is the most effective deterrent from migrants seeking asylum.

In March, leaders of the Congressional Hispanic Caucus asked the White House for a meeting with all agency heads who contribute to the immigration policies that resulted in family separations, saying various leaders have given contradictory accounts of how the separations were carried out.

On Monday morning, CNN’s Jeffrey Toobin said Nielsen’s part in family separations would be her political legacy and that she would be remembered as “the woman who put children in cages.”

A portion of Nielsen’s testimony before the House Homeland Security Committee in which she argues over whether the conditions in which migrant children are detained can be considered “cages” went viral last month.