WASHINGTON — Russian President Vladimir Putin offered President Donald Trump his praise. Defense Secretary Jim Mattis offered his resignation. America's allies fear the worst. And the top foreign policy lawmaker in Congress cannot stop laughing.

As a quarter of the federal government barrels headlong into a shutdown, the biggest story in Washington concerns America's presence overseas, where allies fear a retreat of the global superpower that has been the bedrock of military and diplomatic alliances that have lasted since the end of the second World War.

On Sunday, Trump tapped Deputy Secretary of Defense Patrick Shanahan to become the acting Secretary of Defense, following Mattis' shock resignation. Shanahan, a former Boeing executive who spearheaded the development of the 787 Dreamliner, took the Pentagon's second ranking role in 2017.

Shanahan "will continue to serve as directed by the president," said U.S. Army Lt. Colonel Joe Buccino, to CNBC on Sunday. "The Department of Defense will remain focused on the defense of the nation."

His temporary appointment appears unlikely to allay the growing concerns about the direction of U.S. foreign policy, or the turmoil surrounding the Trump administration.

"I think you will have extreme consequences," Sen. Ben Cardin, D-MD., told CNBC in a phone interview on Friday, speaking about Mattis's surprise resignation the night before. The retired general's resignation came after Trump's announcement that the U.S. would be pulling troops from Syria, and followed his decision to remove troops from Afghanistan.

That announcement came on the heels of a government shutdown, tumbling markets, and amid lingering uncertainty about the president's legal exposure as special counsel Robert Mueller continues his probe into Russian interference in the 2016 election.

Cardin, the top Democrat who served as ranking member on the Senate's foreign relations panel for most of Trump's presidency, called the former Marine's exit "one of the most difficult changes in the Trump administration."

He added: "Not only his leaving, but the manner in which he has left will create major concerns about the reliability of America as a partner."

Mattis issued a terse letter to the president Thursday evening, in which he hinted that the two had irreconcilable differences about their views of the world, citing their treatment of global alliances in particular. His resignation is effective at the end of February.

"As this Administration continues to implode, Secretary Mattis' extraordinary resignation is a significant loss and a real indication that President Trump's foreign policy agenda has failed and continues to spiral into chaos," Sen. Bob Menendez, the Foreign Relations Committee's ranking member, said in a statement.

Mattis was seen as the lone remaining grownup in Trump's Cabinet, willing to push back against a commander-in-chief who disdains the government's foreign policy apparatus, and has little use for traditional diplomacy.

"Secretary Mattis represents the last of what we might call the mainstream foreign policy thinkers in the Trump administration," said Jim Lindsay, who recently co-authored the book The Empty Throne: America's Abdication of Global Leadership. "His departure is going to shape the balance of advice the president gets. And I think it is also going to change how American foreign policy is viewed overseas."