Richard Engel has been NBC News' chief foreign correspondent since 2008.
An NBC News review of the events surrounding Gen. Qassem Soleimani's killing shows that the U.S. and Iran were closer to war than was generally understood.
The Iraqi Parliament and prime minister requested the removal of U.S. troops, according to a draft letter.
President Donald Trump said Friday that he ordered the drone attack that killed Qassem Soleimani in order to prevent a conflict. But Iran has vowed revenge.
"The great nation of Iran will take revenge for this heinous crime," the Iranian president tweeted.
"His relatives were subjected to harsh treatment by ISIS and he no longer believed in the future of ISIS. He wanted to take revenge on ISIS and al-Baghdadi himself," Gen. Mazloum Abdi told NBC News.
The informant proved to U.S. intelligence that he had direct access to Baghdadi this summer by turning over the ISIS leader's used underwear and a sample of his blood.
U.S. forces have been pulling out of northern Syria amid growing chaos after Turkey invaded the region earlier this month.
"I want him to stay committed to his words," the head of the Kurdish-led Syrian Democratic Forces said of President Donald Trump.
Many of the prisoners who spoke to NBC News appeared to know little of the chaos that was unfolding around them beyond the confines of their compound.
Yuriy Lutsenko said he was friends with the president's personal attorney and that the pair had known each other for "many years."