We were both deeply involved in our nationâ€™s fight against al-Qaida and its early fight against the Islamic State, and we both believe it is imperative that President Donald Trump remember what we consider to be the most important lesson of counterterrorism - the ability of a terrorist group to quickly regenerate if the pressure on it is reduced. The pattern is clear: If we do not keep the pressure on the Islamic State and other groups, we believe we will again face a threat to our homeland.
We saw this pattern repeatedly during our careers - always with potentially devastating consequences. When the United States drove al-Qaida out of Afghanistan in the fall of 2001, the group went to prearranged safe houses in the urban areas of Pakistan, where it continued to conduct aggressive attack planning. Over the next year, the CIA, with the assistance of Pakistani intelligence, systematically captured many al-Qaida members, including a number of the groupâ€™s senior leaders, and significantly reduced the groupâ€™s capabilities to attack us.
The remnants of al-Qaida then moved again, this time in 2003 to tribal areas of Pakistan - in a remote region along the Afghanistan-Pakistan border where the Pakistani government has limited control, and where the group was no longer under pressure from the U.S. or Pakistani governments.
Without that pressure, the groupâ€™s capabilities to attack us rebounded. By the summer of 2005, al-Qaida directed and organized the July 2005 London bombings. And by the summer of 2006, al-Qaida was able again to catastrophically attack the United States.
A similar dynamic took place in Yemen. In the aftermath of 9/11, an al-Qaida-associated group there grew in size and threat - the outgrowth of a senior al-Qaida leader fleeing South Asia for the Arabian Peninsula. It conducted several attacks. The United States, working with the Yemeni government, was able to bring the group under control by arresting and imprisoning most of its leadership. But then a massive prison break in February 2006 allowed the group to re-establish itself.
Within 24 months of the prison break, al-Qaida in the Arabian Peninsula - or AQAP, as it is known - was again a threat to the West, conducting two attacks on the U.S. Embassy in Sanaa in 2008, nearly bringing down a U.S. airliner flying to Detroit on Christmas Day in 2009, coming close to detonating sophisticated explosives packed in printer cartridges on two U.S. cargo flights in 2010, and nearly putting an individual wearing a nonmetallic suicide vest on a U.S. aircraft in 2011.
The Obama administration acted quickly, weakening the group with a military campaign, an operation that continues today.
The Islamic State was, in part, an outgrowth of the withdrawal of U.S. forces from Iraq at the end of 2011. When the U.S. military departed the country, al-Qaida in Iraq (AQI), the predecessor organization to the Islamic State, was largely - although not completely - defeated, the work of the Bush and Obama administrations, along with the support of the Iraqi government. When we left, there were only a few thousand AQI members remaining, most of them in hiding.
Without U.S. military support, AQI began to bounce back almost immediately. What does this key lesson mean for today? It means the United States and its coalition must keep the pressure on the tens of thousands of Islamic State members who remain in Iraq and Syria. It means we need to do the same in Afghanistan, where the remnants of al-Qaida work closely with the resurgent Taliban. And it means we must keep the pressure on al-Qaida in the Arabian Peninsula in Yemen.
Michael Morell, a Post contributing columnist, was deputy director of the CIA from 2010 to 2013.