In February 176, the United States began construction of Fort Liberty at EM1. A smaller non-military station had been built by the US in 179, and the PRC, with Russian assistance, had finished construction of their Tianmen station at EM1 the next year. Fort Liberty was perceived by the PRC to be a great threat, due to its shipyard and long range missile systems, and Dominio privateers were engaged to disrupt construction supplies being shipped from orbit. In response the US heavily funded Constellis ship construction. By 174 BFC Constellis corvettes were escorting all supply ships and had had a number of skirmishes with privateers. Both the US and PRC fleets remained in orbit or lunar transfers, as did the Constellis largest ship, the newly purchased CS Justice, of similar size to a post-war cruiser although lacking longer range missiles. North Korea did not have any non-orbital military space assets, but engaged in multiple remote sabotage attempts on US ground based and orbital construction facilities.

On the 19th March 176, with the station nearing completion, the US sent the Cruiser USS Unity on a Freedom of Navigation exercise to Fort Liberty. It refuelled at the station and was on its return on the 23rd March. At 16:30 that day the USNS Herrick came under fire from a privateer while delivering supplies. Its two Constellis escort corvettes CS Drake and CS Thor returned fire, crippling the ship, killing its crew, and making it the first ship destroyed in space combat. The Justice was already in a low-energy lunar transfer orbit and began a series of burns to join the Unity, which was adjusting its orbit to intercept Tianmen Station. Before they could join up the Unity was crippled by the Russian missile carrier MRK Mirazh and the commercial PRC drone cargo ship Tianzhou, leaving the Justice isolated and outnumbered with no nearby support and forcing it to use its remaining fuel to vector towards Earth.

During this time both the US second fleet and the PRC rapid response vessels had refuelled and moved out of their normal lunar orbits onto an approach with EM1. The US second fleet consisted of two pre-war carriers and their destroyer escorts, and the PRC rapid response consisted of two cruisers, six fast frigates, and three Russian gunships. The US moved to DEFCON 2 and declared that EM1 was under blockade, but did not declare war. The bulk of the PRC and US fleets were stationed in Earth orbit, where both countries had a No First-Strike doctrine for orbital warfare due to the risk of creating a Kessler cascade. Both fleets dispatched task forces towards EM1, which was several days away from Earth with fission thermal engines.

The Battle of the Apex occurred when the US second fleet engaged with the PRCs rapid response vessels, along with the commercial PRC drone cargo ship Tianzhou and a Dominio privateer, in orbit around EM1. The Russian gunships and the privateer were destroyed in the first hours by the US carrier launched drones, but these were denied access to the PRC fleet by their drone screen. The debris from the battle quickly cascaded and made maneuvering difficult, as well as greatly slowing down the EM1 three body intercept calculations. Both sides quickly ran out of maneuvering hydrogen and had to use their reserves to break off towards their respective stations.

At 02:30 on the 24th, 10 hours after the outbreak of hostilities, the PRC and US agreed to a truce at EM1. There were a number of proxy conflicts in the next few days such as the conflict between the Mirazh and Justice, but none directly between the US and PRC. A total of 11 vessels were destroyed in the conflict, with 98 crew casualties. As a direct result of the conflict, the PRC, Russia, and North Korea created a mutual defense treaty stating that an attack on any member or their colony was a collective act of war against all, this treaty was the basis of the GPR alliance. Both the GPR and the US made a number of changes to their fleets, most notably increasing the number of supply vessels and smaller high maneuver ships transported into battle by large heavy burning carriers. Doctrine also changed to emphasise laying down and screening a much greater density of fuel depots, and using point defense lasers to clear debris. The effectiveness of swarms of attack and screening drones was well understood before the war, but both sides launched new fast high density drone carrier variants after it.

Ten-Hour War