It is the eighteenth title in the Ace Combat series. In an alternate reality 1994, the 5254 Ulysses collided with a fictional moon of Jupiter named Polyphemus. The collision resulted in a mass of 10,000 meteors that were expected to impact the Earth in five years. The world's countries decided to construct several anti-air railgun networks named "Stonehenge", which would destroy the asteroids before they made impact with the Earth. In July 1999, the meteors approached Earth. The railgun network was ineffective against the large number of meteors, and the impact event ended in a catastrophic loss of life. About 20% of the world was reported to be destroyed.
20 years later, in 2019, a conflict breaks out between an organization known as the United Nations Forces (UNF) and another unknown faction.
Source: Wikipedia, "Ace Combat Infinity," available under the CC-BY-SA License.