Imagine with me, if you will. You are leader of a small but prosperous empire, controlling roughly two dozen small star systems. You and your allies have been at war with the most powerful civilization for years, and you've managed to gain the upper hand. A long and costly war has cost you several star systems and most of your fleet. Finally, they agree to peace. You sign the papers, and begin to focus on internal affairs.
As you check your intelligence briefings, you notice that another empire has been building up their military. Significantly. As it stands, the Yor have four to five times as much firepower as the rest of the nations put together, as well as a much larger population. They begin to greedily prey upon the weaker nations, demanding tribute or conquering them.
You halt all social and military production, and begin pouring all of your resources into your R&D projects, developing weapons and shields specialized to counter the growing threat. One by one, your allied nations fall to swarms of powerful Yor ships.
After the research had advanced sufficiently, you empty your coffers and upgrade your small fleet, and purchase some additional cruisers. You position your fleet on the Terran-Yor border and wait.
Its time. Your destroyers flash across the border, seeking out the numerically superior Yor fleets and engaging them. Terran ships begin to explode, but the Yor are technologically inferior, and they explode faster. War erupts across the galaxy as civilizations pick their sides. A galactic conflagration begins.
Behind your destroyers, fleets of heavily-escorted troop carriers descend onto Yor worlds, crammed with billions of troops. Tens, then hundreds, of billions die as your begin to inch your way towards the Yor homeworlds. Your war machine keeps turning, producing dozens of sophisticated warships.
Eventually, Icos itself falls. You survey your vast empire, won with the blood of billions and billions. Your huge fleet dwarfs all others. Your allies are afraid of you. You've positioned fleets on all of your borders, back up with troop ships.
Then you realize: you've become what you set out to destroy.
This is Galactic Civilizations.