The Middle East is entering what many analysts see as a dangerous new phase. With the Islamic State group on the brink of defeat, the long-simmering rivalry between Saudi Arabia and Iran threatens to boil over, with Lebanon in the crosshairs.
It was a resignation like no other and it is still sending shockwaves through the region.
He made his announcement not from Lebanon but in Saudi Arabia, the country that acts as his political backer. Many Lebanese believe he was pushed into the decision by Riyadh.
It is still not clear when, or if, Mr Hariri will return home.
The spectacle of the missing prime minister is being seen as part of the wider regional struggle between Sunni-ruled Saudi Arabia and Shia-dominated Iran.
For now, Lebanon is uncomfortably centre stage - it is after all where proxy wars have been fought in the past.
Iran backs the Shia movement Hezbollah here. Its supporters believe Mr Hariri's resignation was orchestrated by the Saudis in order to weaken their influence in the country.
Hezbollah has been accused of operating a "state within a state". Its armed wing is more powerful than the Lebanese army and it leads a bloc which dominates the cabinet.
On Thursday, Saudi Arabia and its Gulf allies further ratcheted up the pressure by urging their citizens to leave Lebanon, sending a clear signal of a toughening up of its policy towards the country.
"The Americans, the Saudis, the Israelis are all trying to prevent Hezbollah from maximising its gains from the wars in Syria and Iraq," says Hassan Ileik, an editor at the pro-Hezbollah newspaper, Al Akhbar.
"What is happening in Yemen is also related to the Lebanon situation. Hezbollah and its allies have achieved enormous success. But they're now facing huge pressure because of this."
Saudi Arabia has accused Hezbollah of firing an Iranian-made missile at it from Yemen, where Riyadh says Iran is also equipping Shia rebels it is leading a long war against. Iran denies the claim.
Basem Shabb is a Lebanese parliamentarian from Mr Hariri's political bloc. He says that the influence of Iran and its allies need to be checked.
"As the situation in Syria comes to an end the regime has the upper hand," he says. "Iran and Hezbollah are seeking dividends in Lebanon for the role they played in Syria."
"Because this has a regional dimension the solution is not going to come from within Lebanon. The more powerful actors who are interested in stability will need to intervene with the local players to help us maintain stability."
'On the brink'
Meddling in Lebanon's affairs by great powers is nothing new. But the fear is a misstep now could trigger something far graver.
"In the last few decades, we've never been so close to the precipice," warns Maha Yahya, director of the Carnegie Middle East Center think-tank.
"The threat of a regional war has never been this real where a conflict would involve a variety of different countries."
And that is why what happens in Lebanon matters to us all.
The so-called Islamic State group is all but defeated. What is happening now though - the growing rivalry between Saudi Arabia and Iran - could be even more dangerous for the region and beyond.