The reapers have returned from dark space. With Earth under attack and the mass relays in a state of chaos from refugees and the councils fleet engaging reaper forces the galaxy is a mess. The outcome is unknown, the warfare is devastating. The galaxy will never be the same.
Cerberus being a splinter group of the Alliance started off with Alliance cruiser designs but bumped to a more aggressive design but not without still have a few of these ships commissioned for defense of key military installations.