On May 7, 2018, Secretary Nielsen, despite her objection, officially enacted a controversial practice of the Trump administration's policy of separating parents and children accused of crossing over the U.S.–Mexico border illegally.

At a congressional hearing on May 15, 2018, Nielsen testified that she would enforce the then-newly enacted Trump administration policy of separating parents and children who crossed over the U.S.–Mexico border, noting that similar separations happened in criminal courts "every day."

In June 2018, Nielsen stated that the Trump administration did not maintain a policy of separating migrant families at the Southern border; The Washington Post fact-checker described Nielsen's claim as false and "Orwellian."
At that point, the Trump administration had in six weeks separated approximately 2,000 migrant children from their parents.
Contrary to Nielsen's claims, the DHS website showed that a policy of family separation was in place.

On June 18, 2018, Nielsen defended the policy at a sheriffs' conference but said the administration had asked Congress "to allow us to keep families together while they are detained" as an alternative. "
We cannot detain children with their parents so we must either release both the parents and the children – this is the historic 'get out of jail free' practice of the previous administration – or the adult and the minor will be separated as the result of prosecuting the adult.
Those are the only two options.
Surely it is the beginning of the unraveling of democracy when the body who makes the laws, rather than changing them, asks the body who enforces the laws not to enforce the laws.
That cannot be the answer."
Three days earlier, the DHS said that it had separated 1,995 immigrant children from 1,940 adults, which it described as "alleged adult parents," at the border between mid-April and the end of May. Because the law forbids children from being kept in criminal detention facilities, they are separated from their parents.

Nielsen held a press briefing with White House spokesperson Sarah Huckabee Sanders in June 2018 amid growing public outcry about the family separation policy.
Nielsen accused the media and members of Congress of mischaracterizing the administration's policy.
She dismissed the suggestion that the administration was using family separations as political leverage to force Congress to support Trump's broader immigration agenda or to deter migrants from coming to the United States.
In doing so, she contradicted comments made earlier by Attorney General Jeff Sessions, Chief of Staff John Kelly and senior adviser Stephen Miller.
She got very little support from administration officials such as Miller, who was openly against her.
John Kelly, who had strongly recommended her to Trump, was her biggest advocate amongst the people who talk to Trump the most.

On June 20, 2018, after repeatedly arguing that the administration could not sign an executive order to end family separations, she was present at Trump's signing of an executive order ending his "zero-tolerance" policy of separating of children from families.
Sources told Politico that Nielsen had privately pushed for this executive order behind the scenes while at the same time saying publicly that the executive order could not be created.

In September 2018, The Intercept reported that Nielsen had previously personally authorized the family separation policy after receiving an April 23, 2018, memo by the heads of three federal immigration agencies (U.S. Citizenship and Immigration Services, Customs and Border Protection, and Immigration and Customs Enforcement) recommending the family separation policy for the express purpose of deterring migration.
At the time of the report, Nielsen had avoided attributing deterrence as the purpose of the policy.

In 2019, Sen. Jeff Merkley (D-Ore.)
wrote in a letter to FBI director Christopher A. Wray: “...the FBI should immediately investigate whether Secretary Nielsen’s statements [to Congress] violate 18 U.S. Code § 1621, 18 U.S.C § 1001, or any other relevant federal
On May 7, 2018, Secretary Nielsen, despite her objection, officially enacted a controversial practice of the Trump administration's policy of separating parents and children accused of crossing over the U.S.–Mexico border illegally.
At a congressional hearing on May 15, 2018, Nielsen testified that she would enforce the then-newly enacted Trump administration policy of separating parents and children who crossed over the U.S.–Mexico border, noting that similar separations happened in criminal courts "every day."
In June 2018, Nielsen stated that the Trump administration did not maintain a policy of separating migrant families at the Southern border; The Washington Post fact-checker described Nielsen's claim as false and "Orwellian." At that point, the Trump administration had in six weeks separated approximately 2,000 migrant children from their parents. Contrary to Nielsen's claims, the DHS website showed that a policy of family separation was in place.
On June 18, 2018, Nielsen defended the policy at a sheriffs' conference but said the administration had asked Congress "to allow us to keep families together while they are detained" as an alternative. "We cannot detain children with their parents so we must either release both the parents and the children – this is the historic 'get out of jail free' practice of the previous administration – or the adult and the minor will be separated as the result of prosecuting the adult. Those are the only two options. Surely it is the beginning of the unraveling of democracy when the body who makes the laws, rather than changing them, asks the body who enforces the laws not to enforce the laws. That cannot be the answer." Three days earlier, the DHS said that it had separated 1,995 immigrant children from 1,940 adults, which it described as "alleged adult parents," at the border between mid-April and the end of May. Because the law forbids children from being kept in criminal detention facilities, they are separated from their parents.
Nielsen held a press briefing with White House spokesperson Sarah Huckabee Sanders in June 2018 amid growing public outcry about the family separation policy. Nielsen accused the media and members of Congress of mischaracterizing the administration's policy. She dismissed the suggestion that the administration was using family separations as political leverage to force Congress to support Trump's broader immigration agenda or to deter migrants from coming to the United States. In doing so, she contradicted comments made earlier by Attorney General Jeff Sessions, Chief of Staff John Kelly and senior adviser Stephen Miller. She got very little support from administration officials such as Miller, who was openly against her. John Kelly, who had strongly recommended her to Trump, was her biggest advocate amongst the people who talk to Trump the most.
On June 20, 2018, after repeatedly arguing that the administration could not sign an executive order to end family separations, she was present at Trump's signing of an executive order ending his "zero-tolerance" policy of separating of children from families. Sources told Politico that Nielsen had privately pushed for this executive order behind the scenes while at the same time saying publicly that the executive order could not be created.
In September 2018, The Intercept reported that Nielsen had previously personally authorized the family separation policy after receiving an April 23, 2018, memo by the heads of three federal immigration agencies (U.S. Citizenship and Immigration Services, Customs and Border Protection, and Immigration and Customs Enforcement) recommending the family separation policy for the express purpose of deterring migration. At the time of the report, Nielsen had avoided attributing deterrence as the purpose of the policy.
In 2019, Sen. Jeff Merkley (D-Ore.) wrote in a letter to FBI director Christopher A. Wray: “...the FBI should immediately investigate whether Secretary Nielsen’s statements [to Congress] violate 18 U.S. Code § 1621, 18 U.S.C § 1001, or any other relevant federal 
Kirstjen Nielsen