One of the most harmful mistakes of Barack Obama’s administration will be his deliberate and politically motivated underestimation of the terrorist threat. He was re-elected in 2012 partly on his bold assertion that al-Qaida was “on the run.” Osama bin Laden was dead, so Obama declared the fight against al-Qaida over, much like he did in Iraq, and he walked away.

Donald Trump thus inherits a bucket of craziness with Iraq (and the Middle East in general), but he will need to get wise about al-Qaida right away. Terrorist watchers in the military and among private contractors largely believe that al-Qaida is on the rebound.

Taking out Osama bin Laden and a string of high-level commanders in Iraq, Africa and Asia severely disrupted the terrorist group’s ability to operate. They lost territory, support and their ability to communicate with one another. This, if you will remember, was one of the Bush administration’s instructions for going after al-Qaida way back in 2001. Neutralize them so they can no longer do any damage. Then go in for the kill.

Obama skipped the kill part, choosing instead to walk away from Iraq, from the threat of al-Qaida, all of it. But he didn’t miss the opportunity to create a campaign narrative that made him strong on foreign policy — in spite of even Benghazi — long enough to win re-election. Sadly, it worked.

Obama’s actions in Iraq and regarding al-Qaida are the worst foreign policy decisions of his administration. Leaving Iraq early spun it into chaos at the very moment it was beginning to stabilize.

Al-Qaida laid low when the focus of attention swung to the Islamic State. The wannabe caliphate cut a huge swath through the Middle East, seizing territory, massacring nonbelievers and taking the media by storm.

While ISIL called down the thunder of U.S. and allied air strikes, al-Qaida got back to work, building networks and connecting with local populations. The group’s strategy comes straight from bin Laden, and it calls for getting involved with local groups, spreading the message of jihad, and essentially converting them to the cause. It can be a long process, but for a culture that thinks in terms of centuries, not weeks, it’s easy to see why al-Qaida isn’t quitting.

History will be the final judge of just how bad Obama’s decisions were, or how much damage will ultimately be done. But Trump has to deal with the blowback now.

Al-Qaida is still a force to be dealt with, in large part because Obama chose to declare victory, walk away from the conflict and move on to other things. While Obama was never the commander in chief to make a courageous, tough call for American security, it’s a good bet that Trump is more vested in America’s national security.