White House National Security Adviser Michael Flynn resigned Monday amid a snowballing controversy over whether he lied about his contacts with a Russian official, throwing President Donald Trump’s security team into turmoil just weeks into his term.

Flynn’s resignation came as Trump struggles to cement his national security apparatus as the president and his cabinet officials are preparing for a series of meetings and summits with foreign leaders in the coming months, starting this week in Europe.

Retired Army Lt. Gen. Keith Kellogg, Flynn’s chief of staff, was named acting national security adviser, White House press secretary Sean Spicer said in a statement announcing the resignation.

Along with Kellogg, the White House is considering retired Vice Adm. Robert Harward and former CIA director David Petraeus as permanent replacements for Flynn. None of the three has a history with the president like Flynn’s, who was an early Trump supporter and ardent campaigner.

Amid the disruption, the U.S. is confronting serious challenges on two strategic fronts: the Middle East and Asia. Trump has yet to articulate a plan for combating Islamic State and other radical Islamists that he’s said are the No. 1 threat to the U.S. In Asia, North Korea has tested the new administration by launching a ballistic missile while Trump was meeting Prime Minister Shinzo Abe of Japan, the main U.S. ally in the region. China also is asserting itself, forcing Trump to back down from the notion of using Taiwan as a bargaining chip in dealing with the world’s second biggest economy.

Though Trump campaigned primarily on domestic issues, national security was a central element of his message and it’s been dominant in the early days of his presidency. Following on his meeting with Abe in Washington, Trump is set to meet Wednesday with Israeli Prime Minister Benjamin Netanyahu. Vice President Mike Pence and other top administration officials, meanwhile, are heading at the end of the week to the annual Munich Security Conference, where European allies are desperate for any clues to Trump’s intentions.

If Trump were to tap Harward, it could further empower Defense Secretary James Mattis, under whom Harward served as a deputy commander of U.S. Central Command. Harward served on the National Security Council’s staff and at the National Counterterrorism Center under President George W. Bush. He was a Navy SEAL and commanded forces in Iraq and Afghanistan. Harward is now chief executive of Lockheed Martin United Arab Emirates.

Former Vice Adm. Robert Harward

Petraeus is well known as a retired four-star general lauded for his leadership in the wars in Afghanistan and Iraq. But his ability to repair the reputation of Trump’s National Security Council may be compromised by another matter that brought him notoriety: a 2015 plea deal including two years of probation for a misdemeanor, after he shared classified documents with a biographer with whom he had an extramarital affair. The episode forced his resignation as CIA director under President Barack Obama. He was under consideration by Trump for the secretary of state job that ultimately went to Rex Tillerson.

Former CIA Director David Petreaus

Trump might hope to use Flynn’s departure to put an end to the questions about whether Flynn had improper contact with Russia.

“This seemed inevitable when it became clear Flynn had misled Vice President Pence over the calls,” said Brendan Thomas-Noone, a research fellow at the University of Sydney’s United States Studies Centre. “Trump’s administration needs to distance itself from the suspicion it has close links with Russia and this may give it some space.”

But Democrats were emboldened by Flynn’s departure and plan to press for more details.

Retired Army Lt. Gen. Keith Kellogg, acting national security adviser

“The reality is General Flynn was unfit to be the National Security Advisor, and should have been dismissed three weeks ago,” Reps. John Conyers of Michigan and Elijah Cummings of Maryland, the top-ranking Democrats on the House Judiciary and Oversight committees, wrote in a joint statement.

“Now, we in Congress need to know who authorized his actions, permitted them, and continued to let him have access to our most sensitive national security information despite knowing these risks,” they wrote. “We need to know who else within the White House is a current and ongoing risk to our national security.”

Rep. Eliot Engel of New York, the top Foreign Affairs Committee Democrat, called for a “thorough, bipartisan investigation to get the complete picture of Russia’s interference” in the U.S. presidential election following Flynn’s resignation.

Flynn still had some Republican support. House Intelligence Committee Chairman Devin Nunes of California said in a statement that Flynn “deserves America’s gratitude and respect for dedicating so much of his life to strengthening our national security.”

The burgeoning scandal forced Trump to choose between an adviser who had been with him almost since the beginning of his presidential campaign and his vice president, who gave a public defense of Flynn based on what he’d been told by the national security adviser.

Flynn said in his resignation letter that he had “inadvertently” misinformed Pence about his contacts with Russia because of “the fast pace of events.” Flynn’s tenure as national security adviser was the shortest in the history of the post, Thomas-Noone said.

Pence’s standing in Congress, where he served in the House from 2001 to 2013, meant that what had been a firm line of Republicans standing behind Trump and Flynn was likely to break as Flynn’s story fell apart.

Throughout the day Monday, Democrats were calling for Flynn’s ouster. All the Democrats on the House Oversight Committee signed a letter demanding an investigation. There were cracks forming among Republicans before Flynn resigned. Sen. Marco Rubio of Florida told reporters that Flynn’s Russia contacts should be investigated by the Senate Intelligence Committee as part of an investigation of Russian interference in the election, and Rep. Mike Coffman of Colorado said Flynn should resign if he misled the president.

An administration official said the White House has been reviewing Flynn’s actions for several weeks in light of a Justice Department warning about his contacts with the Russian ambassador to the U.S., Sergey Kislyak.

The department informed the White House last month that Flynn had discussed U.S. sanctions with the Russian envoy and misled officials about the conversation, according to a U.S. law enforcement official with knowledge of the matter.

The warning about Flynn was delivered to the White House counsel’s office by then-acting Attorney General Sally Yates, a career prosecutor who was held over from the Obama administration, said the official who requested anonymity to speak about the sensitive matter.

The Washington Post reported earlier the warning and that Yates was concerned Flynn was potentially vulnerable to being blackmailed by Russia, an account that the official confirmed. Trump fired Yates after she refused to defend his executive order banning travel from seven predominately Muslim nations.

White House officials sent conflicting signals about how much the revelation had damaged Flynn. On Sunday, Trump senior policy adviser Stephen Miller declined to defend Flynn or say whether his job was safe. Miller, appearing on NBC’s “Meet the Press,” said Flynn had served the country admirably but that “it’s not for me to tell you what’s in the president’s mind.”

As late as 4 p.m. on Monday Trump counselor Kellyanne Conway said in a television interview that Flynn “does enjoy the full confidence of the president.”

Just over an hour later, Spicer released a statement that painted a different portrait. Trump “is speaking to Vice President Pence relative to the conversation the vice president had with General Flynn and also speaking to various other people about what he considers the single most important subject there is, our national security.”

With assistance from Chris Strohm Jason Scott and Steven T. Dennis