Lethal threat remains after killing of Islamic State leader

Eliminating the Islamic State group’s elusive leader gives President Trump a new argument for leaving Syria, but the U.S. military campaign against the extremists is far from finished.

The killing of Abu Bakr al-Baghdadi by U.S. forces leaves the Islamic State without an obvious leader, a major setback for an organization that in March was forced by American troops and Kurdish forces out of the last portion of its self-declared caliphate, which once spanned a swath of Iraq and Syria.

But the militant group, which arose from the remnants of al Qaeda in Iraq after that group’s defeat by U.S.-led forces in 2008, has ambitions to regenerate. And it remains a dangerous threat in Iraq, Afghanistan and beyond.

“The bottom line is: This puts the enemy on its heels, but the ideology — and this sounds so cliched — it is not dead,” said Chris Costa, a former senior director for counterterrorism for the National Security Council in the Trump administration.

Key to the Islamic State is its “kill where you are” ethos, encouraging a far-flung network of followers, including those in the United States, to commit violence however and wherever they can. That jihadist message is likely to live on, even with the death of al-Baghdadi.

That means U.S. forces, perhaps in reduced numbers, will continue hunting and attacking key Islamic State targets, even as Trump says he’s committed to a 2016 campaign pledge to bring them home and end “endless wars” started under his predecessors.

Trump earlier this month went from declaring a near-complete withdrawal of U.S. forces from Syria to deciding that some, perhaps several hundred, must stay to keep eastern Syria’s oil fields from falling back into the hands of the Islamic State. Trump also agreed to keep about 150 U.S....