At 8:46 on a bright New York City morning Sept. 11, 2001, a passenger airliner plowed into the North Tower of the World Trade Center. At 9:03, a second passenger jet hit the South Tower. The world then watched the inconceivable as, less than 90 minutes later, both towers collapsed.

At 9:37 that morning 18 years ago, another hijacked airliner slammed into the Pentagon. At 10:03, a fourth plane, its apparent target the White House or U.S. Capitol, crashed into a southern Pennsylvania field, forced down by courageous passengers who had learned their nation was under attack and their plane one of the weapons.

Nearly 3,000 people were killed. If not for the heroic actions of fire, police and other first responders to clear the towers, thousands of more would have died. Those heroes who survived the rescue continue to die from illnesses contracted at the smoldering ruins.

The 9/11 Commission formed to analyze how this could have happened concluded, “The 9/11 attacks were a shock, but they should not have come as a surprise.”

Warning signs were missed. Poor communication among domestic, intelligence and defense personnel, up to the highest levels of government — both in the administration of President Bill Clinton and that of President George W. Bush, in office at the time — contributed to the failure to foresee the potential attack, concluded the commission.

“The most important failure,” the panel concluded, “was one of imagination.”

Because planners had not envisioned such a domestic assault they were poorly prepared to recognize the evolving plot. The failure to coordinate intelligence gathered by different agencies contributed to the nation’s vulnerability.

This is a reminder that the nation cannot be satisfied with preventing a repeat attack — airport security has been dramatically amped up since Sept. 11, 2001 — but must have the imagination, preparation and communication to recognize the next attack, the one in manner and scope not yet seen.

Then there was this from the commission: “No president can promise that a catastrophic attack like 9/11 will not happen again. But the American people are entitled to expect that officials will have realistic objectives, clear guidance, and effective organization.”

The continuing upheaval seen in the administration of President Trump, most unsettling in the leadership positions responsible for protecting the nation, does not provide confidence that there are agreed upon objectives, clear guidance or effective organization.

On the eve of the 9/11 anniversary, Trump fired his national security adviser John Bolton.

We are no fan of Mr. Bolton’s and, in an editorial following his appointment in the Spring of 2018, noted the oddity of the pick.

“The president ran as an America first isolationist, saying the country must focus on problems here at home rather than trying to fix problems globally. Yet he named as his new national security advisor John Bolton, a neocon who has strongly advocated using U.S. military might to influence global events,” we noted.

In dismissing Bolton, the president seemed to have finally recognized they were a mismatched couple. Bolton had apparently made it known he disagreed with Trump’s continuing courtship of Kim Jong-un, the brutal North Korean dictator, even as Kim refuses to surrender his nuclear program and thumbs his nose with repeated missile firings.

Bolton reportedly was also at odds with Trump’s overtures to use such a personal approach in trying to broker deals with Iran and the leadership of the Taliban terrorist group in Afghanistan.

While hardly sad to see the hawkish Bolton go, the lack of clarity it suggests in the administration’s approach to national security is disconcerting. Trump has now gone through three national security advisors in less than three years in office.

Consider also that Homeland Security Secretary Kirstjen Nielsen resigned in April. Kevin McAleenan was named as acting secretary, but Trump has yet to make an official appointment.

In July, Dan Coats, who had a reputation for being willing to challenge the president, resigned as the director of national intelligence, a position created on the recommendation of the 9/11 Commission to address intelligence failures in advance of the attacks.

That vital position is also filled with an “acting” leader, Joseph Maguire, former director of the National Counterterrorism Center. A solid pick, Maguire is hindered by the president’s failure to say whether he will make his appointment permanent. Trump initially had tapped Rep. John Ratcliffe, a staunch political supporter of the president, but the nomination was withdrawn when both Senate Democrats and Republicans pointed to concerns about Ratcliffe’s inexperience and seemingly blind loyalty.

“Agreed upon objectives, clear guidance, effective organization” — the administration is striking out on all three.

The Day editorial board meets with political, business and community leaders to formulate editorial viewpoints. It is composed of President and Publisher Timothy Dwyer, Executive Editor Izaskun E. Larraneta, Owen Poole, copy editor, and Lisa McGinley, retired deputy managing editor. The board operates independently from The Day newsroom.