As the last memory of The Old Republic, the Nova Empire had taken on the responsibility of maintaining diplomacy throughout Known Space. Lying in a strategic and pivotal location in the galaxy, it was able to keep various aggressive species, such as the Romulans, Klingons, and Kree from expanding into defenseless worlds.
When Thanos brought together the Chitauri, First Order, Mandalorians, Ravagers, Sinestro Corps, and the Sith Order in his attempt to gain the Infinity Stones, much of the governments in the Local Group became concerned that the Mad Titan would engulf the galaxy into warfare. In response. Xandar and the Nova Corps sought out an alliance for mutual protection and aid.Meeting on the planet Babel, representatives from the Nova Empire, Shi'ar Protectorate, the Tellarite Alliance, and the Vulcan Confederacy formed the Coalition of Planets which provided a framework for both military and civilian cooperation. It was instrumental in dealing with the various incursions by Thanos on their systems.
However, the Coalition's real test was a humanitarian one, aiding Earth after the terrible assault by the forces of Apokolips. With millions dead and billions without basic needs, the various member worlds devoted themselves to protecting and helping this backwater planet which had become so important in recent years.
Though the destruction of Xandar by the Warworld and the fragmentation of the Nova Empire seemed destined to spell the end of the Coalition, it has had the opposite effect. The other members of the Coalition helped defend Empire worlds from the Kree, Romulans, and Klingons. The unity shown by the member nations encouraged the Minbari Triune to sign the treaty, and the Andorians even negotiated a peace treaty with the Vulcans in order to join.
With the Coalition of Planets growing within the galaxy, a multi-planet group has suggested a more integrated and permanent alliance be negotiated. So far the idea of the United Planets is still an optimistic dream, but its proponents believe that continued support and cooperation will make their vision a reality.