The Potemkin Ship
The concept of a decoy ship isn't new, and in the Cosmic Era, it is easy for a drone to have the appearance of a full sized, fully manned warship. The uses of decoy ships are many, but this isn't about a decoy ship, this is an entirely different kind of ship. A Potemkin ship is a ship that was never real, but the vast majority of people believe the ship to be real.
The Eurasian Alliance battlecruiser Matushenko is a Potemkin ship. Superficially it is a cruiser of moderate size with large engines and impressive firepower, plus a wing of integrated aerospace fighters. The ship has had several crews, and has done a number of tours around the solar system, and participated in a number of naval stand-offs, most notably over the Russian colony of Astrokrasny on Mars. The ship has minimal crew, high amounts of automation, and only minimal weaponry and armor. The crews of marines and green sailors who've served on the ship have done patrols and painted corridors, and think they are on a real warship. Matushenko is a large cargo ship with military grade engines and an outer shell designed to look like a battlecruiser. The deception of the Matushenko was discovered after a collision with a damaged and unpowered Neo-Soviet supply ship Sinop. The collision tore a massive hole in the bow, destroyed three gun turrets and collapsed the forward command spire, revealing the truth of the Matushenko. The ship was lost with more than 300 casualties.
The discovery of the Matushenko sent shockwaves through fleet commands across the solar system. The ship had been in service for almost two decades and has eluded being exposed. Examination of fleet records showed that the ship had been present at many encounters, but it was always 3rd or 4th in the line, a filler, to make the Neo-Soviet forces look more intimidating.
The 'Year of Lances' followed the revelation of the Matushenko and involved various faction's ships taking pot shots at each other, nothing intended to be lethal, but powerful enough to reveal which ships were authentic and which were decoys. This proved to be problematic as several actual warships were damaged, and came close to starting international events, and a number of ships were discovered to be frauds. The Year of Lances culminated with the Sarajevo Event, where an independent mercenary ship, the eponymous Sarajevo. The ship engaged the Federation battlestar Odyssey, attempting to put a shot through the bow. There was a strong belief that the Federation's vaunted battlestars were themselves all Potemkins, given their size, power, and the fact that they had almost no service in their service record. The Odyssey engaged the Sarajevo and utterly destroyed the ship with a single volley from the forward batteries.
A popular use for a Potemkin ship is to be involved in a false flag operation, and these come in two forms. The first has a ship that emulates a foreign power's ship and uses it to strike a friendly target to build outrage and public support for military and economic action against said foreign power. The second is for the Potemkin ship to be destroyed or badly damaged, seemingly at the hands of a foreign power.
The EUdAS ship Bolivar, a frigate, was involved in a South American attack on New Themyscira and the Caribbean League faction of the Atlantic Federation. The frigate destroyed two Amazonian monitors and a dozen mecha before falling back. The Federation backed a Themysciran initiative that saw economic sanctions against the EUdAS as well as several punitive raids against the continent. The only problem with this conflict was that at the time of the attack on the Amazon island, the Bolivar was above Patagonia, assisting with tracking Antarctic pirates. One of the targets of Federation reprisals was the Bolivar itself, which was headhunted and sunk by classified assets. It is popularly believed that the Bolivar that attacked New Themyscira was a Potemkin ship, and that the actual Bolivar was deliberately destroyed to prevent it from being examined by investigators after the conflict.
The SAUR light cargo ship Hendrikkson was attacked by pirates above the Horn of Africa, and was destroyed with all hands lost. This was blamed on pirates, but there was a strong undercurrent that the pirates operated with the blessing of the Federation, as said pirates never bothered Federation assets (seeing as almost all of them were heavily protected). The loss of the Hendrikkson inflamed the South Africans, and saw a resurgence in support for the military as well as the decision to shelter Amerikka Command's warships outside of North America. There was no outbreak of hostilities, but there was public support for building new ships and bolstering the military to better protect said interests. There was no actual cargo ship named the Hendrikkson, and it is believed that the ship was a badly aged fuel ship that was moved into the area and 'attacked' and sunk to sell a narrative.
Additional Ideas (1)
The Potemkin Fort is a military installation that is high profile, and apparent very high value of some sort.
This is a patent falsehood, and the Potemkin Fort is an actor. It draws the attention of foreign powers, and is the subject of espionage and in a wartime scenario is one of the first targets for destruction. To this end, the fort is a 'Honey Pot' designed to draw in and waste enemy assets and resources, or it is a pure decoy. By attacking the Potemkin fort, a real installation is spared destruction. This gives defenders time to rally and strike back, or even use the fort as an ambush point
During the Resource Wars, the United States had a clandestine research facility in the Aleutian Islands that was involved in beam and high energy weapons research. Fort Jericho was known by several world powers, and when the combined Russian and Chinese strike force attacked the island in their Operation Dzhakuzi, Whirlpool, they were destroyed. Jericho housed a Directed Energy Weapon, and when the task force attacked, the remotely controlled facility activated, and fired the weapon. Over a dozen warships were sunk and tens of thousands of red troops were killed. The loss was such that there was not another serious attempt on the American west coast.