ONYX covered the US Iraq withdrawal on September 30 (Day 54): the 23-year arc; Iran and its allies welcoming the departure; Iran’s specific strategic gains. Today, October 1, the withdrawal is confirmed complete. The specific addition in today’s reporting: the departure has raised concerns about a security vacuum and the possible strengthening of Iran-backed militias.

THE SECURITY VACUUM  

A security vacuum occurs when a military or police force withdraws from a geographic area faster than a replacement force can establish effective control. The specific conditions for a security vacuum in post-US Iraq:

▸  Iraqi Security Forces: trained and equipped with US support for two decades; their specific capacity to maintain order without US advisory and air support is a documented uncertainty

▸  Popular Mobilization Forces (PMF): Iran-backed militias that are formally incorporated into the Iraqi state structure but maintain separate command chains responsive to Iranian guidance

▸  ISIS remnants: dispersed but not eliminated; ONYX documented the CENTCOM counterterrorism mission as one of the US’s remaining operational justifications; without US advisory support, Iraqi counterterrorism capacity may be reduced

▸  Kurdish north: the Kurdistan Regional Government has its own security forces; their relationship with Baghdad and with Iran-backed forces is a documented tension point

THE IRAN-BACKED MILITIA DIMENSION  

The PMF — Popular Mobilization Forces — are the specific actor whose strengthening is documented as the primary concern:

▸  The PMF contains dozens of militia groups, the most powerful of which — Kataib Hezbollah, Asaib Ahl al-Haq, and the Badr Organisation — maintain direct ties to Iran’s Islamic Revolutionary Guard Corps

▸  With US forces no longer present, the PMF faces less operational constraint

▸  The PMF’s documented strategic goal: consolidation of political and military power in Baghdad

▸  The regional land corridor Iran seeks — Tehran through Iraq and Syria to Lebanon — becomes more feasible without US forces disrupting militia logistics

THE HORMUZ CONNECTION  

The US force withdrawal from Iraq in the same week as the Hormuz five-day countdown is the arc’s most analytically significant single geopolitical timing event since the dual-base strikes on September 13. The US military presence in Iraq has been a documented component of the regional military balance that has been the context for the Hormuz negotiations. Whether its removal changes Iran’s negotiating calculus on Hormuz is the specific question the arc tracks without asserting a connection that is not confirmed.

The US spent 23 years in Iraq. American troops left. Iran and its allies celebrated. The security vacuum that follows is being filled by the forces that have been waiting for this. Iran-backed militias that have operated alongside but constrained by US forces can now operate without that constraint. Five days to October 6. The same Iran whose allies are celebrating the US departure is the one negotiating Hormuz through Qatar. Both are documented simultaneously.

WHAT HAPPENS NEXT  

▸  PMF operational expansion — whether Iran-backed militias formally expand their territory and activities

▸  Baghdad political realignment — whether the government formally shifts orientation toward Iran

▸  ISIS counterterrorism — whether any ISIS resurgence occurs in the absence of US advisory support

▸  Regional land corridor — whether Iran consolidates its Iraq-Syria-Lebanon connection

CONFIDENCE: HIGH

American troops completing withdrawal; remaining Iraqi bases departure celebrated. Iran allies raised concerns that a security vacuum is possible, strengthening Iran-backed militias, according to confirmed reporting.

⚖️  BIAS CHECK — WHO IS SAYING WHAT
Iran and allies	Celebrating; the withdrawal serves their documented strategic interests in Iraqi political influence and the regional land corridor
Iraqi Security Forces	Inheriting the security responsibility; their specific capacity is the documented uncertainty
PMF / Iran-backed militias	The strengthening actors; their specific operational plans are from developing reporting
US military	Has completed a presence that cost 4,500+ American lives; the withdrawal was requested by the Iraqi government
ONYX	Covering the security vacuum dimension not present in September 30; naming the Hormuz timing without asserting causal connection

SOURCES

▸  Confirmed reporting — US Iraq withdrawal complete; security vacuum October 1, 2026