Stephen Miller is reportedly behind Kirstjen Nielsen's exit from DHS

There is no love lost between President Trump and Homeland Security Secretary Kirstjen Nielsen, who announced her resignation on Sunday.

Trump has long been frustrated with Nielsen, and vice versa, people with knowledge of the situation told NBC News. Nielsen became head of DHS in December 2017, and her biggest defender in the administration was her predecessor, former White House Chief of Staff John Kelly. Last May, it was reported that Trump berated her during a meeting, angry over the rising number of undocumented migrants crossing the southern border. Nielsen drafted a resignation letter, but chose not to submit it.

In November, just days after the midterm elections, Trump told advisers he wanted Nielsen out, The Washington Post reported. He allegedly didn't like it when she tried to talk to him about immigration laws and regulations, and would often complain that she wasn't tough enough. On Friday, there was more tension between the two when Nielsen found out that Trump had decided to withdraw the nomination of Ronald Vitiello as director of Immigration and Customs Enforcement, without notifying Nielsen, a person close to the secretary told NBC News.

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Kelly is now gone from the White House, and Nielsen doesn't have a lot of supporters in the administration. A senior official told CBS News that Trump's hardline adviser, Stephen Miller, is orchestrating an overhaul of the Department of Homeland Security, and Nielsen's departure is part of it. Throughout her tenure, Nielsen defended the administration's immigration policies, including separating families and shutting down the government in an attempt to get money for Trump's border wall.