The unprecedented resignation of a Secretary of Defense as a repudiation of an American Commander-in-Chief’s worldview and damaging impact on Washington’s global alliances marks the latest top US administration official to resign in protest of “Trumpism.” At this juncture, many in Washington and capitals worldwide are nervous about the implications, given that Secretary of Defense Jim Mattis served as a check on some of President Donald Trump’s most dangerous instincts and tendencies. Mattis has served as a mediator who helped preserve other NATO members’ trust in America, possibly stopped Trump from bombing North Korea last year, and opposed a US policy of regime change in Iran.

For Washington’s Gulf Cooperation Council (GCC) allies, Mattis’ decision to resign effective Feb. 2019 is unsettling because the Pentagon chief’s restraining influence over Trump has benefited the Gulf monarchies throughout the unpredictable Trump presidency. Without Mattis restraining the president, GCC states are likely to feel more vulnerable to chaos severely exacerbated by the Trump administration’s unpredictable actions and failure to devise a long-term strategic and coherent vision for America’s role in the greater Middle East.

Trump’s style of announcing decisions via Twitter without consulting allies beforehand on major issues of mutual interest is unsettling and a source of concern for allies, especially those like Arab states in the Gulf that rely on the US as a security guarantor. In Doha, for example, there is widespread recognition that both Mattis as well as then-Secretary of State Rex Tillerson played pivotal roles in preventing the blockade of Qatar from escalating into (what could have been) a military confrontation in mid-2017.

With Mattis at the helm of the Pentagon, the Qataris have had a trusted partner in the Marine General. He assured Doha of Washington’s continued commitment to remaining Qatar’s security guarantor in the face of external threats. In the immediate aftermath of Trump’s first tweets about the GCC dispute, which suggested that his White House would back Riyadh and Abu Dhabi over Qatar, Mattis among other US officials put out a fire that had much potential to severely undermine Washington’s interests in the Gulf and beyond.

In the case of Oman, Mattis’ visit to Muscat to meet with Sultan Qaboos in March helped soothe some of the tension in US-Oman relations that resulted from Trump’s decision to pay tribute to all GCC states in his historic speech and meet with their leaders, except Oman, in Riyadh in May 2017.

Looking ahead to 2019, Trump’s foreign policy may well be more geared toward his re-election bid with domestic considerations being even more heavily on the president’s mind than before. Gulf leaders are aware that Trump, more so than any of his predecessors, makes major decisions on the international stage that are connected to his domestic political interests. For example, in key swing states that will be key political backgrounds in the 2020 election, many American voters in the center of the political spectrum see little reason for US military forces to remain in Afghanistan for more than 17 years and have little appetite for continuing to pay the bills for that campaign which began the month after the September 11, 2001 attacks.

Against the backdrop of such concerns, GCC officials are aware that the next Pentagon chief will likely lack Mattis’ experience and background in the Gulf as the head of U.S. Central Command from 2010 to 2013. As a gifted mediator between Trump and Gulf officials who possessed a strong understanding of the Middle East’s security dilemmas from a GCC perspective, Mattis’ resignation is dramatically unsettling leaders of Washington’s close allies in the Arabian Peninsula which have concerns about problems with the Trump administration that could quickly stem from the absence of a reliable and competent interlocutor like Mattis.

As stated in Mattis’ resignation letter, the failure of this US president to realize the shortcomings of unilateralism and disregard for historic alliances have undermined America’s ability to advance its vital national interests. Whereas Trump’s populist instincts, praise for authoritarian leaders in Brazil, Poland, Egypt, the Philippines, etc., and support for mercantilist trade measures have played well with certain domestic audiences that back Trump in the States, Mattis was a strong figure in the administration respected by allies and taken seriously by foes worldwide at a time when the current US leadership constantly relinquishes opportunities to lead internationally.

On issues from climate change to trade and human rights to resolving conflicts plaguing the Middle East, which all heavily concern the regimes and societies of the Gulf region, the Trump administration has diminished America’s standing and influence as a global power that can bring nations together. Mattis’ resignation is merely the latest indicator of this trend that decreases GCC states’ confidence in America as a reliable leader and security guarantor for the long haul.

For Gulf monarchies, attempting to better comprehend Washington’s unpredictable foreign policy in the Trump era will probably prove even more challenging without Mattis in the administration. A major fear is that Trump’s nationalist and populist politics will even more heavily shape the White House’s rhetoric and policies vis-à-vis the Middle East with negative implications for the future of US-GCC relations.