Here is my story (and I am sticking with it). The player's people are from Earth, but a slightly different history of Earth (so I can play a few games about when something was invented, why, by whom, and not get piles of Pedantic letters from amateur historians about why I GOT IT ALL WRONG.)
The key pivotal point where this story's timeline diverges from our own comes down to an obscure decision by a person largely lost to history in a battle that nobody knew was going to be important. That battle: The Battle of the Brandywine. The Date: September 11, 1777. The person: Captain Patrick Ferguson.
Patrick Ferguson was a sharpshooter for the British Army. At one point in the battle, he had a clean shot on General Washington, but for personal reasons, elected not to take it. In the timeline of this game, he did take the shot. And the world doesn't remember George Washington as the General that led the USA through the Revolution. They remember General Nathanial Greene (Washington's then second in Command.)
And, because Washington was not in charge at other key parts of the Revolutionary War, we don't get bungling like losing the Battle of Germantown, having to camp in Valley Forge, the british occupation of Philadelphia, and so on. We get completely different bungling. However, Franklin was still machinating with the French. The American alliance with France (particularly in the department of Naval Support) still is ultimately what wins the war.
Because I picked an event that happened largely before the Industrial Revolution, I can play games with the order in which things happened to make the timelines in my story work.
In the Game Earth, they only had one World War, and it starts earlier in history (1904), but goes until the same date in our timeline (1918). The war ends in stalemate. Between casualties of the war and the spanish flu none of the sides in the war is able to field an offensive for nearly a generation.
During this Earth's War, the German Empire conquers the Russian Empire, but immediately after the war collapses into a Soviet/Nazi mashup: Krasnovia.
But before anyone could go off and start what would have been their World War II, ballistic missile technology and the atom bomb were developed. Basically all of the energy our Earth put into airplanes and cars, their Earth put into rocket and nuclear technology.
A cold war developed between Krasnovia and pretty much everyone else. Part of the insanity of a prolonged cold war was a proxy war for the colonization of space. Krasnovia couldn't expand out, so it expanded up.
The Krasnovians got in all of the early punches in the race for space. They effectively control the Moon, economically and militarily. They have a massive Navy. Their craft are fast. Their weapons are hard hitting. They have built all of their holdings on the Moon to repel any kind of invasion by any other power. Their forces are overwhelming. But the reach of those forces is largely confined to the Moon and to a lesser extent high Earth orbit. High performance, hard hitting spacecraft don't have a lot of extra mass budget for fuel and provisions.
To counter the threat of Krasnovian dominance of space, pretty much everyone else in the world got together to form the The International Space Treaty Organization (ISTO). By the time ISTO was assembled, Krasnovia had unquestioned control of the Moon. ISTO had to aim further out. The ISTO targeted the Asteroid Belt. Their colonies took a lot longer to establish, and require fundamentally different technology than the Krasnovians developed to conquer the moon.
If you are wondering about Mars, I have a separate blog post on the topic: Why nobody colonized Mars
Krasnovia's moon colonies are closer to Earth, and have more material to work with. But, because Earth is a short hop away, they didn't develop the fundamental technologies that would be required to sustain life indefinitely in space. Their life support technologies are largely extensions of the technologies we are familiar with in our time. They grow food staples for a bulk of their calories, but items like meat and fruit are shipped from Earth. Their life support equipment also has vital components and expendables that are only manufactured on Earth. This allows Krasnovia to exert control over their colonies in the event of a rebellion.
ISTO focused less on engine performance and weapon yield, and instead focused on fuel efficiency and self-sufficiency. They are especially good at using one colony as a stepping stone to build the next. Their Naval Vessels are not so much warships as swiss-army-knives that can bolt on weapons. Virtually every weapon system they have is actually a dual purpose system with a civilian application.
The events in our game are taking place in the 2070s-2080s. The ground breaking technology has opened the game wider: The G-Drive. A G-drive allows a vessel to accellerate a 9.8m/s/s (1g) virtually indefinitely. The technology has lead to another space race, this time to establish dominance of surrounding star systems.
Krasnovia got in the early punches because they developed the drive first. They managed to reach Alpha Centauri. The problem is that all of their technology is tied back to Earth. The range of their ships is limited by the shelf life of key nutritional elements in food (about 10 years). A G-Ship can travel 11 light years in that time. In practice, to account for slippage, that radius is much shorter. Krasnovia has an outpost in the Sirius System, (8.6 ly away) but its ability to sustain the outpost in the long term with their current logistics chain is questionable.
Because ISTO's technology has been built from day 1 to assume some sort of naval blockade could prevent communications from Earth, all of their colonies are completely self-sustaining. While the range of Krasnovian ships in deep space is limited by the shelf life of provisions, ISTO vessels are limited only by fuel. And you can stop virtually anywhere in the Galaxy for more. This self-reliance has a price. ISTO vessels are much, much larger and more expensive to build. They essentially have to pack en entire ecosystem on board.
ISTO's strategy is send colony ships far enough away that they will have a few decades of a head start before Krasnovian forces could reach them. (Presumably after Krasnovia gets over its control issues and replicates ISTO's ecosystem technology.) ISTO has selected star systems that have minor planets and/or an asteroid belt that can be economically exploited. And with those resources they plan on establishing more massive settlements, which can, in turn, send out more colony ships.
And if you follow all of those leaps in logic, the design of the Iliad makes sense. I'm really trying hard to build some solid foundations for the story, so I can avoid the massive amounts of Retcon that seem to pollute all of my favorite Sci-Fi television experiences growing up.