Forty years ago the US embassy in Yemen was a tower house in the heart of downtown Sanaa. Mud-brick and beige, it blended into the neighbourhood, which in 1972 also included the Soviet Embassy. At the time Sanaa was little more than a village; 50,000 people inside walls that had stood for centuries. The revolution that had begun a decade earlier was finally sputtering to an end after years of brutal fighting, and most Yemenis alive today had yet to be born.
In the intervening years the promises of that revolution faded and gave way to another one. One more ruler has been overthrown, and the US Embassy is now a fortified compound on the edge of town.
The rest of Sanaa, meanwhile, has doubled, tripled and then doubled again, sprawling its way towards becoming a filthy holding cell for more than 2 million. Layers of hastily constructed buildings continue to pop up, wrapping themselves around the kernel-of-old Sanaa like some sort of out-of-control snake eager to squeeze the life out of a city the Prophet Muhammad once called "the paradise of earthly paradises".
It is here, in these battered neighbourhoods of bare concrete and exposed wires, where a new and often-ignored generation of Yemeni men has been shaped.
These are the young men known throughout the Arab world as "shabab". They tend to travel in jostling packs of testosterone and barely concealed anger. In Yemen, these are the young men you see chewing qat at 10am instead of waiting until the afternoon as Yemenis have for centuries. And, last week they were the ones who overran the US embassy.
Many of the attackers appear to have come from Musayk, a crowded cluster of poverty nestled below where the US embassy now stands. When their fathers were born the whole area was little more than an empty slope where travellers rested their camels before entering Sanaa. Now, one generation on, it is a bifurcated world of private generators and privilege set off against their dismal world of absence. Nights in Musayk tend to be stifling and dark as the neighbourhood suffers through one of Sanaa's routine electric outages.
Outside their windows, the hill above them is a different world with whirring air conditioners and twinkling lights that never fail.
Only yards away, the heavily fortified US embassy, thick-walled and surrounded by Yemeni soldiers, is out of bounds; no one from Musayk will ever be issued a visa. Farther up the hill is the Sheraton Hotel, another forbidden zone, which for years was one of the few places in Sanaa one could legally buy alcohol. Much of that traffic has now migrated to the top of the hill, just beyond Britain's new bunker-like embassy, to the Movenpick, a gleaming new hotel and nightspot overlooking the squalor of Musayk far below.
There are dozens of other impoverished neighbourhoods, their poorly paved roads reeking of sweat and diesel fumes, dotted throughout the capital. Musayk is merely the most notorious. In recent years, several of its young men have slid over the narrow edge of gang-violence and into jihad and Al Qaeda-inspired suicide attacks.
But for every self-styled martyr there are dozens more who remain behind, seething and angry in their own inarticulately brutish way. And whenever there is what - within their narrow communities and neighbourhoods - passes for a socially acceptable excuse for violence, they take to the streets. In 2005, it was declining subsidies and rising fuel costs; last week it was defence of the Prophet Muhammad.
This wasn't Al Qaeda. It wasn't the Huthis, or the former president, Ali Abdullah Saleh and his family sticking it to the US. Nor was it current President Abdrabu Mansur Hadi trying to embarrass his old boss, or any of the dozens of other candidates up for role of mastermind. It was much simpler than that, more primal and much more worrisome.
This was men from Musayk and similar neighbourhoods taking pickaxes to embassy windows, burning American flags and carting off whatever they could carry. Cars, which were apparently deemed too big to take, were set on fire and, in one case, crashed into the embassy gate. This was frustration and anger masquerading as a protest.
Yemen isn't the first country to deal with angry young men. But there is a key difference between Yemen's version and what came before. Unlike other times and places, where frustrated young men could eventually opt out either through migration or by getting jobs, Yemenis are stuck. No one wants them. They can't migrate. Politics, even after this latest revolution, is still the carefully guarded territory of a few, and the economy is so bad the few Yemenis with money send it out of the country as fast as it comes in. There are no options. Yemen's angry young men are on a road with no off-ramp.
Thursday's attack wasn't the last mutant offspring of the Arab Spring. It was a glimpse into a world too often ignored, a place where anger erupts into action.
Gregory D Johnsen is the author of The Last Refuge: Yemen, Al Qaeda, and America's War in Arabia, to be published by WW Norton in November
On Twitter: @gregorydjohnsen
