The killing of three US soldiers and the injuring of dozens more in a drone attack in Jordan creates one near-certainty - the United States will retaliate with military force.
The harder question is whether this response will be contained to the "radical Iran-backed militant groups operating in Syria and Iraq" that the White House has accused of carrying out the carnage, or widened to include the regime in Tehran that supports them - or both.
But this foreign crisis is playing into domestic politics as well.
The president's predecessor and arch-rival, Donald Trump - vying to replace him in this year's election - has already used the drone attack to accuse Mr Biden of being weak.
Be in no doubt: the drone strike on Saturday night is a significant escalation in a region that has been threatening to explode ever since the militant group Hamas, also backed by Iran, attacked Israel on 7 October, triggering the Israeli war in Gaza.
Given the fact that US soldiers have died, there is arguably a greater risk of igniting a wider conflict than even the decision by the United States and the UK earlier in the month to launch airstrikes against Iranian-backed Houthi fighters in Yemen.
In a statement, Mr Biden mourned the loss and wounding of the soldiers.
He said facts are still being gathered but "we know it was carried out by radical Iran-backed militant groups operating in Syria and Iraq".
The commander in chief added: "Have no doubt - we will hold all those responsible to account at a time and in a manner our choosing."
The last time tensions between the United States and Iran were this high was in 2019 when Mr Trump was in power and chose to unravel a nuclear deal with Tehran.
At one point, the then president even said the US military had been "cocked and loaded" for a direct strike against Iran in retaliation to the downing of an American surveillance drone.
That strike was called off because Mr Trump said he had felt such a response would have resulted in the deaths of Iranians and that would not have been proportionate given the aircraft that Iran had attacked did not have any personnel on board.
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The drone strike against US forces close to Jordan's northeast border with Syria is very different, though it creates a similar dilemma.
As the attack was allegedly carried out by Iranian proxies rather than Iran directly, it would be harder to justify a direct strike against Tehran unless there is clear evidence of Iranian direction in what happened.
Also weighing into any US calculations will be the need to continue to try to contain the violence in the region - already in danger of intensifying into uncontrolled escalation.
Any direct attack on Iran could trigger retaliatory Iranian missile strikes against US targets in the region or against American allies such as Israel.
Many Gulf states will also be incredibly nervous as they too would potentially be in the Iranian firing line.
By contrast, a decision only to go after the Iranian-backed groups accused of carrying out the strike would probably need to involve a much larger use of force than has happened in the past.
The US has launched airstrikes against Iranian-backed militants in Iraq and Syria on multiple occasions in response to attacks on its troops in both countries.
But this action has clearly failed to deter the threat so far.