Secretary of Defense Jim Mattis’ resignation — apparently on principle — has rocked the U.S. national security community, if not the entire nation. Retired military officers, former cabinet officials, think tank experts and congressional leaders from both parties appear united in their concern about the implications for the nation’s future security.

Mattis, along with fellow generals John Kelly (Trump’s exiting chief of staff) and H.R. McMaster (Trump’s former national security adviser) had been mostly viewed as a source of stabilizing counsel for a president with little to no experience in government or national security affairs. They were the proverbial “adults in the room” — even as their influence appeared to wane. And now they are all gone.

With Mattis’ departure, it is an open question who will advise Trump on issues of national security as the president increasingly acts on impulse rather than expert counsel.

With Mattis’ departure, it is an open question who will advise Trump on issues of national security as the president increasingly acts on impulse rather than expert counsel. This governing strategy is generally concerning, but it is particularly worrisome in the context of military deployments and global alliances. The position of U.S. president requires more than the ability to rile up your domestic base — you must also take your responsibilities as commander in chief seriously. An impetuous or poorly thought out domestic order is bad enough — one that impacts our international security apparatus could start a war.

It had been rumored for some time that Mattis would soon depart the administration. He and Trump disagreed on many major policy issues, as suggested by his uncharacteristically candid resignation letter. Furthermore, it seemed clear that the secretary was frequently not consulted and was even surprised when the president announced significant changes in American security policy. Mattis had testified before Congress that the U.S. should remain in the JCPOA, also known as the Iran nuclear deal, that the president opposed. After Trump stunned South Korea with an agreement to unilaterally end all joint exercises between U.S. and South Korean forces, Mattis reinstated the war games.

Mattis also noted at several points in his resignation letter the critical importance of our allies. But he was forced to devote significant time in the aftermath of the NATO summit to repairing relations with our European counterparts following public disparaging remarks by the president. In many ways, Mattis’ resignation letter was not only a formal protest but also a detailed critique of the president’s policies and worldview.

The apparent final straw was the president’s recent tweet announcing a precipitous American withdrawal from Syria. Mattis, General Joe Dunford (chairman of the Joint Chiefs of Staff), and other senior administration officials have been clear in the past few weeks: An enduring defeat of ISIS has not been achieved. The Pentagon has estimated that thousands of ISIS fighters remained despite the loss of the territory that had comprised the so-called Caliphate. Indeed, the air campaign continues in the region, with over 100 air strikes in the last week.

Furthermore, U.S.-stated objectives in Syria have long included the departure of Iranian forces and proxies as well as the establishment of a political process that would eventually end a civil war that human rights organizations say has resulted in 500,000 Syrian dead and millions of refugees. Sadly, none of these goals have been achieved. Rather, it now appears the United States will leave a vacuum in Syria that will be filled by Russia, Turkey, and Iran to the detriment of our Kurdish allies, Israel and the Syrian people.

But more broadly, what does the Mattis resignation tell us, our allies and our potential adversaries about American national security strategy at the onset of 2019? Recent presidents have often adopted policies that differed from their predecessors and not always agreed with their closest advisers. Still, each adhered to a process that insured relevant advisers and cabinet members were involved, options were vetted and secondary consequences considered. Allied governments were consulted, particularly when they were providing significant military forces to a combined effort. This all occurred within the context of a national security strategy to insure policy choices were aligned with longer-term objectives. That no longer appears to be the case in the current administration. Major decisions are made on Twitter at the apparent whim of the president.

But more broadly, what does the Mattis resignation tell us, our allies and our potential adversaries about American national security strategy at the onset of 2019?

Trump has already announced that the search for a new secretary of defense has begun. Obvious requirements include a broad experience in national security affairs, a deep understanding of geopolitics, a grasp of the workings of the Pentagon and background in the policy process. But a secretary of defense, like any cabinet officer, can only be successful if he or she believes their counsel is valued and welcome by the chief executive.

Ultimately, the next secretary of defense’s primary responsibility must be to speak truth to power. For the security of the nation, he or she must tell Trump what he needs to know even when the president disagrees or does not want to hear it. If the president surrounds himself solely with those who patronize and only tell him what he wants to hear, the American people are poorly served and the nation’s future security is put at risk.