The Dosioan Fleet was a major Battle Group of Musai Cruisers that were known to blockade Earth during the One Year War Era, which prevented the Earth Federation from ferrying troops into space from Earth. The fleet was constantly attacked by Federation remnants that were left in Space from both the Colonies of Side 7 and Luna II, in hopes of wrestling back control of Satellite Orbit, each attack would be stopped by Heirshy.
During the events of the Stalemate on September 26th, UC 0079, the Federation amassed a large fleet from Luna II and moved it to assault the Dosioan Fleet, but the offensive was literally butchered by Heirshy again, causing the remaining ships of the Earth Federation be easily dismantled.
However, by the time of Operation Odessa, the Federation's attacks on the Dosioan Fleet began to increases heavily, to the points that gaps were opened within Satellite orbit allowing some Federation forces to escape and reinforce the Federation remnants in outer space. Heirshy was able to close one of the gaps during this heavy offensive, but was constantly moved between fronts back on Earth from November 7th, to November 10th, UC 0079.
November 10th, would be the final day that Heirshy would be able to defend the fleet, as by that night he would be betrayed by Rambal Ral, and sent to his doom where he would be declared KIA by the Dozle Zabi of Space Fortress Solomon, his death would spark sadness amongst the men of the Dosioan fleet, even though days later Heirshy would return again in order to defend the fleet during another Federation space offensive, this time within the cockpit of a Zaku II.
As the war turned against the Principality of Zeon on Earth following the Federation's success against Operation Odessa, the war slowly started to move more in space, causing more and more of the Dosioan Fleet to lose its grip more and more in Satellite Orbit, to the point that by November 16th, the blockade was opened wide enough for more and more mobile suits to enter space.
By November 17th, UC 0079 Tanaka Marsha attacked the Dosioan Fleet while piloting a GM, where he along with the White Space Crew were then succeffuly in destroying 3 Musai Cruisers further opening a large gap in the Dosioan fleet finally giving the Federation ground in Satellite Orbit.