Everything was going well, when in the autumn of 1248, at 13:43 the 3rd of October, christian launched an unprovoked nuclear strike on Muslim land. Mecca, Shanghai and Delhi went up in flames. Missile silos deep inside the Himalaya counter-striked, erasing Paris, London and Roma Neo from the face of the Earth. Further strikes followed until, at 13:59, there was not a single human being alive on Earth. By the 7th of October, missiles launched from Earth reached the colonies on Mars, erasing any life there as well.
The other forum's report doesn't happen to say it was an unprovoked Muslim attack, does it?
Initial thoughts:
Given the history, I suspect that relations between us and any other survivors are going to be somewhat cold. I suggest we want to defend our holdings well, and if hostilities start try to lure them into a trap and kill them with a combination of our fleet and planetary defenses.
We have a lot of research labs and shipyards, but not much in the way of terrestrial (Titantrial?) industry. I think we should build more factories ASAP, as well as some mines.
We'll need more minerals too, especially corundium. Does this game use geosurvey teams? Regardless, we probably want to send the survey ship out. Assuming the Christians are still on Mercury and might be tempted by a blind, unarmed survey ship, I suggest we send the ship outwards (focusing on the other moons of Saturn first) rather than inwards. We should also probably build a second one, which we can send inwards.
Technology:
Could we get a list of technology and maybe scientists?
I don't speak Arabic (assuming that's what it is), but I am guessing our weapons are meson based, given the range and the fact that PDCs use them. Gunboats with mesons can be pretty effective assuming they don't get swatted out of the sky before they get into range (for this reason as well as not having to pay maintenance, fighters might be slightly better). They're fairly slow for their size, though: I think our first tech goal should be better engines.
Another possibility would be to work on missiles. I don't see any sign of them on our ships, but they're very lethal in this game. For the tactics I'm envisioning (more on this later) we'll want defenses focused around planets; the best planetary defenses are pretty much missiles, fighters, and then gunboats in that order. Our current Meson based PDC is fairly useless, IMHO, though since it costs no maintenance we might as well keep them around and hope they can shoot down a few incoming missiles.
Its been 300 years, so Earth and Mars may be habitable again, but those are far closer to Mercury than to Titan. IIRC, Titan is the only remotely habitable rock in the outer system, so they have an advantage there. We might consider developing jump drive tech and trying to beat them to the stars.
Military thoughts:
Assuming there are still Christians on Mercury, we have to consider them a threat. Being in the inner system, they have some advantages:
The planets are closer together, and they'll be able to establish new colonies faster than us. With decent sensor coverage, they will probably be able to use a single fleet to protect Mars, Earth, Venus, and Mercury, moving to intercept any fleet of ours. In comparison, a colony on Jupiter or Uranus might put the entire inner system between us and our colony. I suggest that our military policy include an element of retaliation rather than just defense; if their fleet heads for Jupiter, our ships probably wont be able to beat them there. But they might be able to beat them to Mercury. On a suicide mission, gunboats could make it two thirds of the way on their own, and they're three times as fast as our full sized ships. In fact, if we go with planet based gunboats it might be best (though cold blooded) to make sure any future designs are capable of a suicide strike on the inner system on their own.
As far as defense, I recommend a strategy of fortified bases. Jupiter and Saturn each have a lot of moons, and a dedicated fighter or missile base on one moon can help defend them all. Gunboats don't need bases, but require maintenance facilities/modules and minerals to maintain. I doubt troops will help much; any war will probably be to destroy the enemy rather than capture objectives, and its a lot easier to nuke troops from orbit than fight them on the ground if you don't care about the planet. We might build some engineers (four would be a nice number, since they would all fit in the troop transport), which can build PDCs on uninhabitable colonies and will offer some defense strength if it comes to a ground fight
We'll also want more deep space tracking stations, so we can keep an eye out for other ships. They're amazingly powerful passive sensors, and they stack without penalty.