The last two decades have taught us that a retaliatory invasion may assuage foreign policy hawks or hard-liners, but without a clear strategy and well-defined objectives, a hasty and ill-conceived response can end up causing more harm than the initial attack.

The U.S. 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-Qaida poised for a comeback.

By attacking the United States 20 years ago Sept. 11 and brutally murdering nearly 3,000 people, al-Qaida leader Osama bin Laden was trying to lure the U.S. military into war in Afghanistan and drain the United States of blood and treasure. Of course, he spectacularly succeeded.

According to the Costs of War project at Brown University, what the U.S. once called the “global war on terrorism” has cost the country $6.4 trillion and the lives of more than 7,000 U.S. service members.

The United States played directly into the hands of bin Laden and his jihadist army when it invaded Afghanistan — where al-Qaida 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-Qaida.

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

Now the U.S. has finally left Afghanistan but without accomplishing its mission of destroying al-Qaida, which is expected to metastasize after the withdrawal.

If jihadists or other terrorist groups manage to successfully mount another major attack on U.S. 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 sizable presence of jihadists — such as Yemen, Somalia or Libya — could trigger a full-scale U.S. 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 U.S. 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 U.S. 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-Qaida senior leaders to justice, President George W. Bush was persuaded to take action.

On Oct. 7, 2001, Bush announced the U.S. military would target “al-Qaida 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 power brokers, ushering in the next phase of the war, which included a U.S. 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 mujahedeen — 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 U.S., the North Atlantic Treaty Organization 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 U.S. repeatedly failed to comprehend.

Twenty years after the launch of the global U.S. counterterrorism campaign, there are nearly four times as many jihadists as there were on Sept. 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 U.S. soil since then, a remarkable achievement that should not be overlooked.

Colin P. Clarke is the director of policy and research at the Soufan Group and a senior research fellow at the Soufan Center.