The US spent trillions of dollars and lost thousands of lives waging war in Afghanistan, and now the country has reverted to its pre-9/11 status, with the Taliban back in power and al-Qaeda poised for a comeback.

By attacking the United States on this day 20 years ago and brutally murdering nearly 3000 people, al-Qaeda leader Osama bin Laden was trying to lure the US into war in Afghanistan and drain it of blood and treasure. He spectacularly succeeded.

According to the Costs of War project at Brown University, what the US once called the "global war on terrorism" has cost the country $US6.4 trillion - and the lives of more than 7000 US service members.

The United States played directly into the hands of bin Laden and his jihadist army when it invaded Afghanistan - where al-Qaeda is headquartered - little more than a month after the 9/11 attacks. Instead of a "war on terrorism", the "war" should have been a narrowly scoped counter-terrorism campaign with a specific target: al-Qaeda.

Without a clear strategy - and relegated to second-tier status after the 2003 US invasion of Iraq - the mission in Afghanistan meandered. What should have been a war to destroy al-Qaeda became an effort to rebuild a war-torn nation.

Now the US has finally left Afghanistan, but without accomplishing its mission of destroying al-Qaeda, which is expected to metastasise after the withdrawal. After so many years of a grinding stalemate with no clear path out of the morass, President Joe Biden vowed to withdraw all US troops from Afghanistan by the end of August and kept his word.

If jihadists or other terrorist groups manage to successfully mount another major attack on US soil, will the United States be able to avoid being drawn into another unwinnable war? If it cannot, an attack planned from a country with a sizeable presence of jihadists - such as Yemen, Somalia or Libya - could trigger a full-scale US intervention, leading to another vicious cycle of occupation, nation-building and isolated successes that never result in strategic victory.

A successful approach to statecraft involves identifying, managing and responding to threats as they emerge. It is about knowing when to use force, and when to show restraint. If the US suffers another devastating terrorist attack, a transparent and nationwide discussion - that includes Congress and other critical government institutions - must occur regarding what the proper response should be.

At the crux of this conversation is whether the US should even have invaded Afghanistan, a question that has taken on added importance, given the outcome of the America's longest war. After the 9/11 attacks, al-Qaida was given sanctuary by the Afghan Taliban, a militant Islamist group that refused to hand over bin Laden to the United States. To bring bin Laden and other al-Qaeda senior leaders to justice, President George W. Bush was persuaded to take action.

On October 7, 2001, Bush announced the US military would target "al-Qaeda terrorist training camps and military installations of the Taliban regime in Afghanistan". Within weeks, the camps were destroyed and the Taliban had been thoroughly routed on the battlefield. The Bush administration refused to consider including the Taliban in a postwar power-sharing agreement with the Afghan government and other powerbrokers, ushering in the next phase of the war, which included a US and allied military occupation of Afghanistan - exactly what bin Laden had wanted.

Bin Laden spoke frequently about his plan to bleed the United States until the point of bankruptcy, as he believed the mujahideen - Muslim fighters who considered themselves "holy warriors" - had done to the Soviets in the lead-up to the end of the Cold War. Afghanistan has long been known as the "graveyard of empires", a land of battle-hardened tribal warriors and unforgiving terrain. Even with the most advanced military in modern times, the US, NATO and allied troops were unable to defeat the Taliban.

Rampant corruption across successive Afghan governments virtually ensured the spread of Taliban "shadow governors", highlighting the political aspect of the conflict, something the US repeatedly failed to comprehend. The US military could destroy any target, but as mission creep set in, navigating complex tribal relations became part of a soldier's job description. Militarily, a cross-border haven in Pakistan meant the Taliban always had a sanctuary where fighters could rest, recuperate and rearm.

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One of the most important lessons from the "global war on terrorism" is that the United States should be modest about what it can achieve with military force alone. Even the world's most powerful military failed in its war to defeat the Taliban and stabilise Afghanistan. Addressing the challenges of failed states and violent extremism requires diplomacy, international development and education for marginalised populations, including women and children.

And yet, the hard power of military threat will continue to be a necessary component of US foreign policy; the threat posed by jihadists remains potent. Given the magnitude of the 9/11 attacks and the psychological impact on the country, the United States had no choice but to respond. But it did so on al-Qaeda's terms, not America's.

Jihadists seized upon the US invasion of Afghanistan and then Iraq in 2003 as proof that bin Laden was right all along - the US was at war with Islam. Several high-profile incidents - including the torture of captured al-Qaeda suspects, the prison for detainees at Guantanamo Bay and the scandal at Abu Ghraib prison in Iraq where Muslim prisoners were ritually degraded, abused and humiliated - sullied American soft power, the ability to lead by example.

Twenty years after the launch of the global US counter-terrorism campaign, there are nearly four times as many jihadists as there were on September 11, 2001. More than 230,000 of these fighters are spread across 70 countries. However, there has never been an attack anywhere near the scale of 9/11 on US soil since then - a remarkable achievement that should not be overlooked.

In the face of major terrorist attacks, nations must be prepared to demonstrate resolve. Revenge can be a powerful elixir, but restraint can be a sign of strength in the face of extreme adversity.