At its narrowest point, the Persian Gulf is only 55 kilometres wide.
One coastline is dotted with US military bases, and the other bank is the border of Iran.
The United States has a substantial military footprint across the Middle East, amassed over decades.
Several permanent bases in the region house thousands of troops, advanced military hardware, US Air Force units and US Navy fleets.
Those bases are on high alert after Iran vowed to retaliate against US strikes on it nuclear sites over the weekend, warning American assets in the region were "legitimate targets".
The concerns were realised on Monday local time, when Iran said it had launched a strike at US bases in the region. No casualties have been reported.
But US officials say they are "fully postured to respond", and any attack on its troops would be a mistake.
Where are the US bases?
Currently, there are at least 19 US military facilities across the Middle East, according to the Council on Foreign Relations.
These have shifted and evolved over the years as security priorities and regional relationships changed.
Eight are considered to be permanent. Among them are sites in Bahrain, Egypt, Iraq, Jordan, Kuwait, Qatar, Saudi Arabia and the United Arab Emirates.
The US also has about 40,000 troops deployed across the region, defence officials said.
[MAP US military presence]Qatar base a 'critical cornerstone'
The US bases are the launch point for air, sea, and intelligence missions.
The Al Udeid Air Base in Qatar, built in 1996, is the largest and most significant in the Middle East.
It was among the sites targeted by Iran.
It covers an area of about 24 hectares.
The base hosts the US Central Command regional headquarters and more than 11,000 US and Coalition service members.
The 379th Air Expeditionary Wing — the largest wing in the Air Force — is stationed at the site, along with hundreds of combat aircraft, tankers, aerial refuelling, and intelligence assets.
Intelligence analysts describe the base as "a critical cornerstone of military operations".
It has supported military operations across the Middle East, North Africa and South Asia, including serving as the primary staging ground for air forces in the campaign against Islamic State (ISIS) militants.
The US began carrying out counterterrorism operations against ISIS across Syria and Iraq in 2014.
The UK also has access to the air base.
Bahrain singled out
The Naval Support Activity facility in Bahrain is home to the US Navy's Fifth Fleet, which includes about 9,000 military personnel and civilian employees.
Its operations cover approximately 6.5 million square km of water, spanning crucial choke points such as the Strait of Hormuz, the Suez Canal and the Strait of Bab el-Mandeb.
After the US dropped more than a dozen of its 30,000-pound (14-tonne) "bunker-buster" bombs on Iran's key underground nuclear facilities on Sunday, Tehran promised to respond.
The US strikes came after a week of open conflict between Israel and Iran, sparked by Israel's sudden barrage of attacks against Iran's nuclear and military structure.
Iranian officials said "any country in the region or elsewhere that is used by American forces" was a target, but Bahrain was singled out.
"Now it is our turn to, without wasting time, as a first step, fire missiles at the US naval fleet in Bahrain and at the same time close the Strait of Hormuz to American, British, German and French ships," Hossein Shariatmadari, a representative of Iranian Supreme Leader Ayatollah Ali Khamenei, said in the hardline Kayhan newspaper.
Iraq and Syria past targets
Before Iran announced it had fired on US military bases, analysts said it was possible that Tehran would follow through with threats to strike US assets in the region.
US bases in Iraq have repeatedly come under attack from Iran and its proxies in recent years.
"Iranian attacks against United States interests in the region appear likely, and perhaps even against the United States," Ben Zala, a senior lecturer in International Relations at Monash University, said.
There are about 2,500 US troops in Iraq as part of the international coalition against ISIS.
They are stationed at various installations, including the Al-Asad and Arbil air bases.
Iranian missile strikes targeted Al-Asad and another US base in Erbil in 2020 in retaliation for the US killing of Iranian General Qassim Soleimani.
There were no reported US deaths, but Pentagon officials said more than 100 troops were diagnosed with brain injuries following the attack.
US forces in Iraq and Syria were also repeatedly targeted by pro-Iran militants following the outbreak of the Gaza war in October 2023.
Iranian-backed militias launched missiles and drones at al-Asad as recently as August.
The US responded with heavy strikes on Tehran-linked targets.
Kuwait in the firing line
Kuwait houses several sprawling military installations, including Camp Arifjan, the forward headquarters of US Army Central.
The Ali Al Salem, known as "The Rock" for its isolated, rugged environment, is roughly 40km from the Iraqi border.
It hosts the 386th Air Expeditionary Wing, which the military describes as the "primary airlift hub and gateway for delivering combat power to joint and coalition forces" in the region.
Facilities in countries such as Qatar, Bahrain, Saudi Arabia, and Kuwait are not usually attacked.
But any nearby country would be an easy target for Iran's retaliation, experts say.
US bases are highly guarded facilities, including air defence systems to protect against missiles and drones.
But even with sophisticated air defences, they would have limited warning time to respond to waves of missile, drone or rocket strikes.
Iran weapons 'far from extinguished'
Iran has spent decades building multi-tiered military capabilities at home and across the region that were at least partly aimed at deterring the US from attacking it.
The country's military infrastructure has been targeted in Israeli strikes over the past week, but analysts say there would still be enough weapons to respond to the US strikes.
"Iran's military capabilities are degraded but far from extinguished," said Jonathan Panikoff, from the Atlantic Council's Middle East Security Initiative.
He added that Iran might also engage with allied groups in the region.
"[Iran] could seek to not only leverage proxies in the Middle East to attack US interests and personnel, but also potentially undertake asymmetric attacks and terrorist attacks against global Israeli, Jewish, or US targets," he said.
The New York Times reported that US military and intelligence officials detected signs that Iran-backed militias were preparing to attack US bases in the Middle East.
Bases in Iraq, and possibly Syria, were named as possible targets.
The chairperson of the US Joint Chiefs of Staff, General Dan Caine, said the US military had increased protection of troops in the region, including in Iraq and Syria.
"Our forces remain on high alert and are fully postured to respond to any Iranian retaliation or proxy attacks, which would be an incredibly poor choice," he said.
US brings in reinforcements
US President Donald Trump has repeatedly warned Iran not to strike any US targets, saying any attack would be met with "the full strength and might of the US Armed Forces".
Since Israel launched its surprise attack on Iran, additional US fighter jets and refuelling tankers have been deployed to the region.
Earlier in the year, Washington also doubled its naval presence in the Middle East.
An additional carrier strike group was deployed due to the threat posed by the Houthi rebels in Yemen.
Since 2023, the Iran-backed militant group has been carrying out attacks on commercial ships in the Red Sea and Gulf of Aden, including strikes against US vessels.
When the extra fleet was deployed, Elizabeth Dent, a senior fellow at the Washington Institute for Near East Policy, said the move signalled two things.
It suggested that the Trump administration was "posturing for sustained, decisive air operations against the Houthis and/or contingency planning in case tensions escalate with Iran," she said.
UN Secretary-General António Guterres has warned that the conflict risks "descending into a rathole of retaliation".
ABC/Wires
