On Tuesday, the Washington Post published a bombshell drawn from a forthcoming book by the journalists Bob Woodward and Robert Costa: during the final days of Donald Trump’s presidency, Gen Mark Milley, the chairman of the joint chiefs of staff and the US’s highest-ranking military officer, called the senior ranking general of the Chinese military and offered to warn him in advance of any American military action against China.

Milley, who reportedly believed that Trump was unstable and might launch a politically motivated military operation, contacted the Chinese without the knowledge of the White House. He no doubt thought that his decision was for the greater good. But if he did indeed negotiate with a foreign military rival without authorization, he violated the longstanding American political tradition that the military is subordinate to elected civilian leaders.

A spokesman for Milley released a statement on Wednesday acknowledging the substance of Woodward and Costa’s reporting, and defending Milley’s actions as “in keeping with [his] duties and responsibilities conveying reassurance in order to maintain strategic stability”.

So far Joe Biden has declined to discipline Milley. On Wednesday, he told reporters that he had “great confidence” in the general. But he shouldn’t. If anything, the president should demand Milley’s immediate resignation.

Milley called the Chinese general Li Zuocheng in October and January. On one call, he reportedly said: “General Li, you and I have known each other for now five years. If we’re going to attack, I’m going to call you ahead of time. It’s not going to be a surprise.”

That alone should be grounds for immediate dismissal. No US military officer can seize authority from the president and call the commanding general of our greatest global rival and tell that general that he’ll be given advance warning of military action ordered by the president. Such an act is dangerously close to treason, as Senator Marco Rubio and others have noted. But Milley deserves to be relieved of duty for far more than this one act of insubordination.

Milley has a troubling, years-long record of poor military judgment and a tendency to subvert his civilian leaders. This is particularly evident in his rosy assessments of the war in Afghanistan and the capabilities of the Afghan military.

In 2013, when Milley was the commander of the International Security Assistance Force Joint Command in Afghanistan, he praised the Afghan forces at a Pentagon press briefing. “This army and this police force have been very, very effective in combat against the insurgents every single day,” Milley gushed. “And I think that’s an important story to be told across the board.”

Yet analysts and observers of the war already knew it was going poorly. Around the time that Milley was praising them, the Brookings Institution’s Vanda Felbab-Brown, for example, argued that Afghan troops “continue to suffer from deeply inadequate logistical, sustainment, and other support capabilities and are also deeply pervaded by corruption, nepotism, and ethnic and patronage fissures”.

It is difficult to say whether Milley knew that the Afghan forces were woefully inadequate, or simply had a glaringly poor ability to assess the progress of a war that he was prosecuting. Either way, he consistently, for years, failed to accurately convey the situation to senior US leaders. But that’s not the worst of it.

In November 2020, according to an investigation by Axios, Trump signed a directive to complete the military withdrawal from Afghanistan by 15 January 2021, five days before the end of his term as president. Milley was “appalled” at the idea of a full withdrawal, according to Axios, and worked behind the scenes to derail the effort. Trump was forced to revise the planned withdrawal deadline to 1 May 2021.

When Trump’s national security adviser, Robert O’Brien, said that the president was going to reduce the troop levels from 4,500 to 2,500 before leaving office, Milley publicly challenged the words of the president’s senior adviser, saying: “I think that Robert O’Brien or anyone else can speculate as they see fit.”

On 21 July 2021, Milley told the Pentagon press corps that despite reports that the Taliban had been capturing scores of districts from government forces, the Afghan forces had “the capacity to sufficiently fight and defend their country”. Less than a month later and mere days after the Taliban had seized Kabul, Milley claimed: “There was nothing that I or anyone else saw that indicated a collapse of this army and this government in 11 days.”

But Milley’s revisionist claim that neither he “nor anyone else” could have foreseen the collapse of the Afghan military is laughable. Just two days earlier, John Sopko, the long-time Special Inspector General for Afghanistan Reconstruction (Sigar), told NPR that the collapse was not in the least surprising:

I mean, we’ve been warning – my little agency – for the last almost 10 years about issues with the … Afghan security forces’ capabilities and sustainment. All the signs have been there. I mean, we’ve been shining a light on it in multiple reports going back to when I started [in] 2012 about changing metrics, about … ghost soldiers who didn’t exist, about poor logistics, about the fact that the Afghans couldn’t sustain what we were giving them. … I think the speed maybe is a little bit of a surprise. But the fact that the [Afghan forces] could not fight on their own should not have been a surprise to anyone.

Moreover, had Milley continued the process of withdrawal that Trump committed to completing by 1 May, the US would almost certainly have been able to finish the mission on time and in relative order – and 13 US service members might still be alive.

The US is not a military junta; it is a republic led by elected civilian leaders, and it cannot allow the creation of any precedent, large or small, of military insubordination.

In 1951, after Gen Douglas MacArthur repeatedly publicly clashed with President Harry Truman’s direction of the Korean war, Truman fired MacArthur. The American public widely considered MacArthur a war hero, and Truman’s decision was bitterly unpopular, but he believed MacArthur had given him no choice. Similarly, in 2010, after a Rolling Stone article depicted Gen Stanley McChrystal and his staff badmouthing President Barack Obama and undermining his command of the war in Afghanistan, Obama summoned McChrystal to Washington and accepted his resignation.

Both decisions were controversial – but they were the right ones, and arguably the only ones, that those presidents could take.

Milley cannot continue serving as the chairman of the joint chiefs of staff. For the good of the nation, Biden must relieve him of duty.

• Daniel L Davis is a senior fellow at Defense Priorities and a former lieutenant colonel in the US army who deployed in combat zones four times. He is the author of The Eleventh Hour in 2020 America