[b][u]The Heavenfall[/u][/b]

The Heavenfall was the name later given to the event that occurred on March 23rd, 2004 at 11:23pm, Greenwich Mean Time. 
An object travelling at a significant proportion of the speed of light entered the solar system from outside the ecliptic plane of Earth, and impacted above the Arctic Circle in central Russia The immediate impact caused massive earthquakes, as well as tidal waves. The shockwave travelled around the globe three times, and shattered windows as far south as Seoul. 
However, this was only the first impact, as a string of follow-on objects laid a path of destruction across Earth's surface, travelling Eastwards, with the final impacts striking in the North Atlantic.
With the projectiles travelling so fast and approaching from such an oblique angle, there had been no opportunity or chance for them to be spotted by the various space agencies of Earth, and even if they had, little time to be able to prepare any kind of credible defence. 

The end result was not quite the end of civilization that had been predicted by popular fiction, but it was very close. 
The impacts caused massive geological upheavals. Earthquakes and tectonic activity rocked the world and toppled buildings, blocked roads, collapsed tunnels and shelters and disrupted and destroyed vital infrastructure and caused widespread devastation. Tidal waves ravaged coastlines and left vast areas flooded, destroying even more vital infrastructure and buildings.
Dust and debris thrown up by the impact darkened the skies and played havoc with weather patterns for months, further disrupting efforts to provide organized response and relief, and hampering rebuilding efforts tremendously. 

With the incredible, unthinkable scale of the devastation, and the inability to respond effectively to the sheer magnitude of devastation, the after-effects of the impacts claimed even more lives, and caused even more destruction themselves. Exposure through lack of shelter, disease, injuries and starvation claimed more lives, along with fires, flooding, and other such disasters and accidents. 
By the time the remaining governments and organised response had finally managed to get into action and begin to provide some kind of relief, the total casualties worldwide were estimated to be in the [i]billions,[/i] roughly a third of Earth's total human population having died as a result of the impact and its' immediate aftereffects. 

The incident became referred to as the 'Day the Heavens fell' in a now-iconic news report, and the name stuck, the 18-month period being referred to as 'The Heavenfall' as a whole, or 'The Long Winter', as a result of the persistent darkness and winter-like weather that ravaged across the world. 
The impact had lasting effects as well. The Earth had been tilted on its' rotational axis, leading to longer summers and shorter winters. The global climate rose rapidly through the greenhouse effect caused by the impact's fallout. This, and the impact being above the arctic circle itself, resulted in the melting of large areas of continental ice and a concurrent rise in global sea levels. Many former coastal cities were abandoned, and capitals or major cities and population centers resettled elsewhere. 

With so much devastation and loss of vital industry, transportation, and other long-held and long-established infrastructure, cultural, economic and civil hubs and sites of great importance, the old order of things and the focus of power shifted. Ancient seats of power such as London, Washington D.C., New York, Tokyo, Los Angeles and others were crippled, and the nations they served heavily affected by the Heavenfall. In the resulting chaos and ruin, other powers were left to pick up the reins of leadership and influence. 
As the impact was in the Northern Hemisphere, the Southern Hemisphere became the new leading seat and focus of power. Relatively speaking; the southern hemisphere was less affected, and much of its' infrastructure had fared moderately better. In other countries, places more inland or at higher altitudes had survived better, and consequently became the new focus of development and population as people flocked to them looking for shelter, and as their intact industries, facilities and infrastructure became increasingly important and vital to the redeveloping world and society.

By default, this upheaval meant the end of the Cold War and of the divide and conflict between the Eastern and Western ideologies. The destruction overall and especially in what had been Russia had left the government, already tenuous and fractured, utterly shattered and dismantled, unable to respond effectively to the crisis that now faced its' people, and that had devastated much of its effective infrastructure and power base. Likewise, the leading nations of the West had been similarly subjected to massive upheaval, and left struggling to rebuild and regain their momentum over the following years. 

[i]Author's Note: The Heavenfall did not completely destroy civilization and society, but shifted the balance of power, and changed the 'modern era' away from the OTL into one where a setting and world like Shattered Steel can exist, while also preserving the idea of modern technology and society, and creating new enemies and powers as enemies and allies to fight against and for. [/i]