The British parliamentary system has its drawbacks. The recent referendum vote to leave the European Union, for example, showed a surprising lack of discipline by leaders of the major Conservative and Labor parties, who were allowed to campaign for “Brexit” without much candor about the consequences, and now many of those who voted to leave regret their decisions.

But the British system does have clear, broadly accepted mechanisms for investigating major foreign and domestic decision-making — as reflected in the painstaking report released last week on Britain's lapdog support for the U.S.-led war in Iraq. The Chilcot report, named for a retired civil servant who oversaw the inquiry, is exhaustive, process-oriented and often redundant. But it is a thorough, honest accounting of a grievous decision made by the British government. The best measure: Families of the 179 British service personnel killed in Iraq have praised it for its candor and integrity.

Meanwhile, more than 13 years after the decision by George W. Bush to launch a reckless war based on lies, falsified intelligence and utterly hapless planning, there has been no such accounting of the causes and responsibility in the United States itself, the principal backer of the war. The failure in this country to undertake a serious, no-stone-unturned report such as this exposes an enduring weakness in our system of government and a truly damaging national trait: a lack of a sense of history and readiness to confront — and learn from — our gravest mistakes.

An “Iraq Study Group” was convened in 2006 to review the mess that developed after the initial overthrow of Saddam Hussein, but its recommendations were feeble and inconclusive. The bipartisan panel can't really be condemned for its effort, however, given that it was assembled after only three years of the war — long before the disastrous consequences became evident.

The Chilcot report, ordered in 2009, concluded that London's decision to go to war was secretly made by the prime minister long before the March 2003 intervention. It said the government's actions were based on questionable legality (without approval by the United Nations) and manipulated intelligence, and without proper military planning and equipment. It also said all diplomatic efforts were far from exhausted in the rush to war.

All of these conclusions are also true of the actions of the 43rd president of the United States, his sidekick, Vice-President Dick Cheney, and their acolytes — in spades. The key missteps by Messrs. Bush and Cheney and company are well known, even if responsibility is less accepted. More important are the disastrous consequences of the invasion and catastrophic post-conflict occupation. They would include:

The deaths of nearly 4,500 American soldiers (about 32,000 injured) and as many as 500,000 Iraqi citizens.

The $2 trillion cost of the war and its aftermath, unbudgeted by Mr. Bush.

The drastic drop in respect for the United States around the world.

The broad use of torture as a recruiting tool for terrorists.

The growth of widespread instability in Iraq and the region, and the fact that Iran, now the dominant influence in a devastated country, was the ultimate winner of the war — if anyone prevailed.

The rush to invade Iraq led to the shift of significant U.S. forces from Afghanistan, further complicating the difficult task of restoring stability to that country.

Our failure to confront the lessons of history is rooted in another, more current malady: a bitterly divided political system — one in which Republican congressional leaders can spend $10 million and more than a year on the tragic, but small-scale, attack on Benghazi, but shy away from examining far more consequential errors. A full-blown inquiry into the Iraq War in this country might not have produced a case for prosecution of those responsible for such a disaster, but it might have shut up the claque of neo-cons who continue to run around crying for more war.

The inability of our political leaders and institutions to go back and examine the causes and responsibility for major failures is somewhat ironic because our military has a time-honored tradition of conducting “lessons learned” studies of all activities — from budget and personnel actions to actual wars.

It seems to be left to our historians to conduct such after-action reports. The latest professional review of the Iraq war has just surfaced in a biography of George W. Bush by the respected presidential historian, Jean Edward Smith. In “Bush,” Mr. Smith argues that the war failed miserably, creating “the conditions for the continuing insurrection” and growth in terrorism. Noting that the invasion was “the worst foreign policy decision ever made by an American president,” he concluded: “Rarely in the history of the United States has the nation been so ill-served as during the presidency of George W. Bush.”

Indeed.

Frederic B. Hill was a foreign correspondent and editorial writer for The Baltimore Sun. Later, as a policy analyst in the State Department, he submitted a detailed letter of dissent to Secretary of State Colin Powell before the March 2003 invasion. He is co-editor, with Stephens Broening, of the forthcoming “The Life of Kings: The Baltimore Sun in the Golden Age of the American Newspaper” (Rowman & Littlefield).