The four men -- all reportedly citizens of Tajikistan -- were arrested on Wednesday in the western state of North Rhine-Westphalia.
The president says he wants to bring home American troops. He keeps resisting diplomacy that could actually secure U.S. interests abroad.
The French president told Trump to "be serious," delivering a real-time fact check of his false claim that most Islamic State fighters are from Europe.
Erdogan warned European leaders they should "revise their stances" toward Turkey, where scores of the militants are imprisoned.
Studies of leadership decapitation suggest groups like ISIS often find ways to survive.
The House speaker demanded that the White House brief lawmakers on the operation, which led to the death of the ISIS leader.
In the graphic account of the so-called Islamic State leader's death, the president described him as having been "whimpering and crying and screaming.”
President Trump said he didn't alert "some" congressional leaders ahead of the U.S. operation that killed ISIS leader Abu Bakr al-Baghdadi.
His death is a major blow, but the extremist group has survived the loss of previous leaders and military setbacks.
"He died like a dog," President Trump said during a news conference announcing the extremist leader's death.