Trump wants  to keep U.S. forces in Iraq so they can “watch” Iran:
President Trump plans to keep United States troops in Iraq to monitor and maintain pressure on neighboring Iran, committing to an American military presence in the region’s war zones even as he moves to withdraw forces from Syria and Afghanistan.
“I want to be able to watch Iran,” Mr. Trump said in an interview aired Sunday on CBS’s “Face the Nation.” “We’re going to keep watching and we’re going to keep seeing and if there’s trouble, if somebody is looking to do nuclear weapons or other things, we’re going to know it before they do.”
An indefinite military presence in Iraq makes no more sense for the U.S. than it does to have one in Syria. Keeping troops in Iraq isn’t going to give the U.S. any knowledge about what Iran’s government is doing inside its borders, and announcing that their mission is an anti-Iranian one exposes them to potential attack from militias aligned with Tehran. Many Iraqis already want U.S. forces out of the country now that ISIS has been dealt with, and there will probably be even more demanding our withdrawal if Trump tries to keep U.S. forces there for this purpose.
Trump’s suggestion that Iran might “do nuclear weapons” is more of the same propaganda that he and his officials have been pushing for months. Iran is unable to develop and build nuclear weapons because it is complying with the nuclear deal that Trump reneged on. Thanks to the nuclear deal, the IAEA is able to conduct very intrusive inspections as part of the most rigorous verification regime, and they would be the first to know if Iran were violating the restrictions set down in the JCPOA.
It is unlikely that the Iraqi government is going to agree to a U.S. presence that is being justified by hostility to its neighbor. Iraq’s president has already said that the U.S. military presence is permitted in the country only for the purposes of counter-terrorism:
#Iraq  President @BarhamSalih  responds to #Trump  statement that US troops in Iraq to spy on Iran: “Don’t burden Iraq with issues that don’t represent a priority for my country.” Said US mil presence based on legal agreement to fight terror, anything beyond needs review. #RCDForum 
— Samya Kullab | سامية كلاب (@samya_kullab) February 4, 2019 
Iraq’s government wants to maintain good relations with Iran, and it isn’t going to go along with an anti-Iranian agenda that can only harm Iraq’s economic and security interests. Many of Iran’s neighbors are not as obsessed with and hostile to Iran as the Trump administration, and Iraq definitely doesn’t want to be a front-line state in some anti-Iranian coalition. Trump’s proposal would needlessly put U.S. troops at greater risk in Iraq, and it would gain the U.S. nothing except more resentment from Iraqis.