When Iraq's new Prime Minister Ali al-Zaidi has talks in the White House on Tuesday, it is highly likely he'll discuss more than one uncomfortable topic. On the table is what is potentially the most uncomfortable subject of all, given the US-Iran war: the role that Iran-affiliated paramilitaries play in Iraq.

Most of the paramilitaries, often grouped together as the Popular Mobilization Front, or PMF, were first formed in 2014 when religious leaders called upon able-bodied men to fight the extremist group known as the "Islamic State." They now number around 240,000 fighters and have an annual budget of about $3.5 billion (€3 billion).

Some — but not all — PMF factions are considered part of Iran's so-called "Axis of Resistance," which also includes Hezbollah in Lebanon, the Houthis in Yemen and Hamas in Gaza. During the ongoing conflict between Iran and the US, certain PMF groups are thought to be behind attacks on US bases in Iraq, Kuwait and Jordan.

Which is why the US wants to see them disarmed.

US Secretary of State Marco Rubio addressed the issue while in the United Arab Emirates last month.