The Solar Navy in the late 02100s

As the 02100s came to a close, the various political groups of Terra began to forge stronger bonds with each other in an attempt to reach out to the stars and to build starships for defending humanity from xenoalien threats, including the Krayrath incursions of the 02190s.  One result of this consolidation was the binding together of the outer Solar System settlements into the Solar Colonies and the creation of the powerful Solar Navy dominated by the offworld powers of Free Aries Colony and the Jovian Republic as well as the UECON TerraFleet.  The two major fleets of humanity would eventually combine together into a single force by the 02220s to face the incursions of the felinoid T'Garans and the ground forces of the Alpha Marines would wage war against the various xeno threats throughout the known worlds.

The Alpha Marines were a group of soldiers whose efforts grew out of the need for dedicated anti-xeno forces.  Using dropships escorted by vertifighter gunships to land troops on various planetary surfaces, the Alpha Marines were key to protecting Terra's interests throughout the known worlds during the 02100s and 02200s.

NorAm and Western Alliance ships served as the first combat ships of the Solar Navy, eventually being phased out in favor of the Terran Solar Cruisers. 

A size comparison of a Solar Navy Mk. III Solar Cruiser and an upgraded Yamato-built Mk. IV Solar Cruiser, both in common use at the turn of the century and the coming of the 2200s.

Another mission of the Solar Navy during this time period was providing ships and ground forces for dealing with the plague of Xenoroaches encountered on several distant colony worlds.  Whether they were the larger brutal tank-like Scarabs or the smaller deadly Rippers, the Xenoroaches were a threat that required the creation of a new Terran force, the Alpha Marines to contain.  

The Alpha Marines were a specialized ground force tasked with fighting against the Xenoroach threat on various worlds throughout the expanding human sphere of influence.  The fact that they were so successful culminated in their becoming the "Protectors of Terra" for the next several thousand years.