Beneath the US decision to designate Iran's Revolutionary Guards as a terrorist organisation is a much bigger battle: the proxy wars of the Middle East.
The current state of the region is largely a consequence of the US decision to invade Iraq in 2003. The Sunni-led dictatorship of Saddam Hussein fell quickly.
But conventional fighting did not end, and instead spiralled into a sectarian battlefield with hundreds of thousands of Iraqis and more than 4,000 American forces killed.
The Shiite leadership that replaced Saddam was a gift to Iran, allowing it to become the foreign power with most influence in Baghdad. Not foreseeing that was arguably the biggest mistake of many among those who planned the invasion.
Iran, as a neighbouring state, intervened at marginal cost. The US on the other hand has endured 16 years of irretrievable costs in Iraq.
Even now the bill is open-ended. The US embassy in Baghdad – the largest diplomatic mission in the world – has had as many as 16,000 staff and contractors spread over 40 hectares acres behind large walls.
Such expense was part of the reason that Donald Trump, as a presidential candidate in 2016, vowed to pull US forces out of the Middle East.
Three years later his aggressive stance against Iran, calibrated to reverse its sprawling activity, could end up pulling America deeper into the region.
The unprecedented designation of the Guards has raised the stakes at a time when the region has fragmented into a complicated patchwork of alliances.
The civil war in neighbouring Syria was in many ways a mirror image of what had happened in Iraq. A regime that leant towards Tehran did not break and now has the upper hand, having crushed rebel forces.
What President Bashar Al Assad lacks but desperately wants is full control of Syrian sovereign territory, leaving open the chances of a diplomatic settlement while at least some American troops remain.
Mr Trump's recent designation has the intention of curbing Iran's involvement in Syria and beyond but it is hard to see how he can do so without raising the risk to his own forces.
His gamble is that America's adversary will back off but history provides many examples to the contrary.
Israel, the main US ally in the region, is already hitting Iranian military targets in Syria. That low-level, sporadic engagement could intensify if Mr Trump's gambit does not work. So what could happen?
Diplomatic channels appear to be closed. The battle between Washington and Tehran is being played out in the open. US sanctions have been met by Iran's counter designation of the US Central Command as a terrorist entity.
The 5,200 American forces in Iraq as military trainers and advisers have a significant footprint.
Attacked in the past by Shiite militias, the US president's stance against their predominantly Iranian backers risks shattering the status quo in which American troops are not yet targets.
Those militias know where US personnel are stationed and could strike their positions. Iraq's politicians, who were previously a liaison point between America and Iran, appear to be bystanders, albeit interested ones.
While Iraqi governments in recent years have considered a US force desirable, particularly after the disaster where ISIS overran local forces in 2014, leaders in Baghdad are listening to Iran more than they are America.
Iraqi Prime Minister Adel Abdul Mahdi recently visited Tehran and Iranian President Hassan Rouhani made a reciprocal trip to Baghdad.
Mr Trump did not venture to the Iraqi capital after visiting US troops at Ain Al Assad Air Base, only about 180 kilometres to the west, in December.
The president's designation of the IRGC was also a rejection of Pentagon and CIA guidance. Both believed it would raise risks for US personnel.
But now that the decision has been taken Mr Trump is moving into a new phase of involvement in the Middle East. His military planners will be preparing a response to any attacks on its forces in Iraq or Syria.
Just as Shiite militias know where US forces are stationed, so does the Pentagon know where its adversaries are based.
Any strikes by one on the other are likely to be matched.
