The resignation of U.S. Defense Secretary James Mattis has prompted alarm around the world over President Donald Trump's national security thinking. Mattis' resignation letter is a scathing attack on the president's thinking and instincts, unprecedented in the modern presidency. The world now waits with mounting anxiety for Trump's next decisions.

Mattis was enjoying retirement at Stanford University when Trump picked him to be his secretary of defense. The president was attracted by several features of Mattis' resume: a distinguished military career that culminated in service as commander of the U.S. Central Command; a fierce reputation, captured by his nickname "Mad Dog"; and notable disagreements with President Barack Obama over priorities in Middle East policy, especially concerning Iran: Any readiness to disagree with his predecessor was a plus in Trump's estimation.

Mattis accepted the offer, motivated by an opportunity to serve his country, the chance to steer a foreign policy neophyte and to again focus his — and his country's — energies on Tehran, which he considered the real source of regional instability. U.S. allies and partners took great comfort from Mattis' presence in the administration, viewing him as a source of continuity amidst Trump's gyrations and declarations. The secretary of defense, along with Chief of Staff John Kelly and Secretary of State Rex Tillerson, were widely viewed as "the adults in the room" — guardrails to contain Trump's more impetuous decisions. It was widely reported a year into the administration that the three men had vowed that one of them would remain in Washington at all times to prevent the president from making a spontaneous move that could be injurious to the national interest.