War of 2015

The War of 2015, also commonly referred to as the Four-month War, the Contact War, the Third Secret War, or simply the Invasion, was a global conflict which is generally recognized to have taken place between March and July of 2015. The entirety of the world's nations were at least nominally involved and coordinated through the combined United Nations Command, seeking to defend the planet from invasion by the Divine Empire of Sirius. While the vast majority of engagements were between conventional military forces and the invaders, the principal strategic fighting force used against the aliens was XCOM, a 'black budget' armed wing of the United Nations Office for Outer Space Affairs formed in the late 20th Century. Virtually no distinction between military and civilian targets were made by the invading forces, resulting in the most vicious period of total war seen in human history.

The War of 2015 was by far the deadliest conflict in Earth's history; most estimates place the total casualties at over 400-600 million lost, around 6% of the total global population prior to the arrival of the first invasion forces on Earth. The heavy focus by the invaders on urban population centers was the main cause of this previously-unheard of level of casualties. While many civilians would themselves not encounter alien combatants during the War, the devastation caused by the fragmentation of the global economy and supply chains led to a dramatic decline in quality of life around the world, causing a significant degree of the deaths to be a result of starvation and disease. Secondary conflicts would also break out on the periphery of the wider invasion between human groups as civil society continued to break down, leading to a rise in subsequent killings as a result of massacres and genocide, as well as extrajudicial killings due to increasing desperation for critical resources in the hardest-hit areas. Human technology was often ill-equipped to have parity with or even defend against the tools used by the invaders, especially psionics, which further compounded deaths.

The reasons for the Elders' invasion of Earth is unclear, but generally believed to be linked to the religious dogma observed within their Empire which seeks out and either integrates, subjugates, or uplifts sapient life, as well as an eschatological obsession with Earth by practitioners of the religion. However, others link the invasion to previous encounters between human military forces and hostile extraterrestrials, seeming to suggest the War and subsequent occupation of Earth as being part of a wider conflict elsewhere in interstellar space. The war began with the arrival of the first "Fog pods" and Sectoids in Cologne, Germany on the 14th of March, 2015. After the Sectoids' abduction of numerous German civilians and engaging with and defeating a detachment of the KSK, XCOM deployed Delta Team, which was subsequently killed as a result of the Sectoids' use of psionics.

On 2 June, XCOM's headquarters in the Mount Weather Complex in Colorado was besieged, ending with the capture of its Commander and scattering of personnel which were able to escape capture or death by the invaders. As a result, XCOM was incapable of undertaking any significant actions throughout the rest of the war, and had effectively ceased to exist. While not the sole armed force deployed by the United Nations in 2015, the loss of Mount Weather had compounded a trend of increasing discord among the members of the joint military force still standing, leading to several nations opting to focus solely on their own territory and airspace, all but withdrawing from the UN. The war was officially concluded on 1 July when a special session of the United Nations General Assembly was called, whereupon they surrendered to the Elders in the presence of their apparent leader, Angelis, accompanied by a Thin Man known as James Wang and United States Ambassador to the United Nations William Thorne. The resulting terms of surrender, known as the Great Accord, went into force immediately after - dissolving the United Nations and replacing it with ADVENT.