NEW YORK  -   John Bolton’s sudden, unexplained threat to use “unrelenting force” against Tehran has raised US-Iran tensions to a new high. But its impact is not confined to these two countries. Like a lethal poison, their mutual enmity is seeping through the veins of an already unstable region that has experienced dangerously high levels of volatility in recent days.

No clear reason was given by Donald Trump’s national security adviser for his decision to advertise the pre-arranged deployment of military reinforcements to the Middle East. But Bolton singled out Iran, and specifically Iran’s non-state allies and proxy forces, as causes for concern. These groups are deeply involved in several conflict zones including Syria, Iraq, Yemen and Gaza, where fighting with Israeli forces re-erupted last week.

“The US is not seeking war with the Iranian regime, but we are fully prepared to respond to any attack whether by proxy, the Islamic Revolutionary Guard Corps [IRGC] or regular Iranian forces,” Bolton said.

Mike Pompeo, the secretary of state and fellow Iran hawk, repeated the warning, vaguely accusing Iran of “escalatory actions”. “If these actions take place, if they do by some third-party proxy, a militia group, Hezbollah, we will hold the Iranian leadership directly accountable,” he said, implying the US may have received intelligence about supposed plots.

This explicit broadening of the range of potential Iran-related US targets is significant. Palestinian Islamic Jihad, a western-designated terrorist group financed by Iran and Lebanon’s Hezbollah, reportedly joined Hamas in firing missiles at Israel from Gaza at the weekend. In a tweet expressing “100% support for Israel”, Trump referred to Islamic Jihad by name.

Maybe Bolton and Pompeo were concerned that Iran was using Gaza or planning some other operation to foment violence against Israel and the US at arm’s length, via its proxies, in retaliation for the recent tightening of sanctions and other hostile American measures.

Syria could provide another clue. Washington chooses to blame Iran more than Russia for the continuing conflict there. The IRGC has built bases and supports Shia militia forces loyal to Bashar al-Assad’s Moscow-backed regime. According to aid workers, recent days have brought numerous regime airstrikes and barrel bombings of hospitals, causing rising civilian casualties and displacements in north-eastern Hama and Idlib.

Whether this is the start of Assad’s long-awaited attempt to reconquer Idlib, the last major rebel-held area, is unclear. But Vladimir Putin, Russia’s president, last week refused to rule it out. Any new offensive would probably involve Iranian-led ground forces. Ever reluctant to criticise Putin, Trump views Syrian violence as another reason to go after Iran.

Frustration in Washington that its Middle East policies are everywhere failing could be another factor. The Saudi bombing campaign in Yemen against Houthi rebels, who are backed by Tehran, has succeeded only in harming civilians and attracting international opprobrium.

 Congressional Democrats have accused Trump of aiding and abetting Saudi war crimes and want to stop arms sales.

In Iraq, the remaining US troop presence is increasingly controversial. Ruling Shia politicians with close links to Tehran, representing majority opinion, have made clear they do not want the country used as an American base against Iran. And despite Washington’s attempts to isolate and sanction it, Lebanon’s heavily armed Hezbollah – Iran’s loyal ally – remains formidable.

Nor can Trump and his advisers rely on traditional allies in efforts to force regime change in Iran. European countries, including Britain, continue to uphold the UN-approved 2015 nuclear pact with Tehran, which the US reneged on last year, while the UN’s nuclear watchdog insists Iran is in compliance.

Trump’s “maximum pressure” has poisoned wells elsewhere, too. China, Turkey and India are expected to ignore US demands to cease buying Iranian oil, despite a 1 May deadline set by Washington. China told the US last month not to interfere in its legitimate and lawful “cooperation” with Iran. In other words, butt out.