• 2390 AD - As the Chirr colonisation fleet approaches Mars' orbit, the Republic of Sol redeploys its own Navy to secure the colonies from potential threat. Surprised that Earth turned out to be inhabited, the Chirr are unsure of the course of action to be taken. Ultimately, the fleet's overzealous commander, knowing about the instability on the Chirr's home planet and under pressure to colonise, orders it to engage, disregarding all protocols. This is the first ever contact with other sentient race for both species. Republican fleet is eventually decimated, with only a few vessels able to escape, and the Chirr soon reign supreme in Earth's orbit, with Orbital Defense Cannons turned offline in fear of potential orbital bombardment, which is estimated to be capable of disabling all surface-to-space defences before the Chirr fleet is crippled. Land contingents are dispatched by the Chirr to invade human Lunar and Martian colonies, and the Human-Chirr War begins. Luna is taken over by the end of the year, but situation on Mars is far more complex. The front soon stagnates, and with neither side having any significant presence in the planet's orbit, and the Republican defenders of Mars choosing not to reveal their hidden assets just yet in hopes of setting up a potential trap, noone is able to break the stalemate.
• 2391 AD - The Chirr land on Earth, the vast devastation still present on the planet due to the Unification Wars as well as subsequent pirate raids allowing them to conducts successfull operations despite being severely outnumbered. The beginning of the "Martian Sword" human counteroffensive in the colonies; full control of Mars eventually regained by the Republic, and small Chirr reinforcements defeated. Luna retaken later the same year.
• 2392 AD - The Chirr blockade the now human-controlled Luna, but hesitate to move any substantial forces there, which allows some human vessels (using Io and Europa's stealth technology) to bypass the blockade and thus encircle the alien ships. Humanity breaks through the Lunar Blockade, forcing it to retreat, but suffers heavy losses. The planned landings on Earth and the opening of a second front there are forced to be pushed back. The Battle of Paris begins, as Chirr forces storm the Spared City.
• 2393 AD - In an all-or-nothing move, the Martian-Lunar forces run the Chirr perimeter in Earth's orbit, exploiting the fact that most of the alien fleet is currently positioned relatively far from the North Pole, and is focused primarily on the regions of the 50th through 40th parallels. Nonetheless, many human ships are sacrificed, but in the end the Chirr largely ignore the landing party - one of the many mistakes the largely non-military Chirr command makes - and most of the Republican forces make it through. More fronts are opened, which proves disastrous for the vastly numerically inferior Chirr.
• 2395 AD - The Chirr fleet in Earth's orbit greatly potentiates their orbital bombardments directed directly on human positions (both military and civilian) - the beginning of the so-called "Fiery Sky". By the end of the year, Republican forces regain full control of Paris.
• 2397 AD - The "Fiery Sky" ends - the Chirr cut their orbital bomabardments to
  near-nothing almost overnight, presumably to save as much payload as possible on a lost cause.
• 2398 AD - Last Chirr forces are expelled from Earth, ending the Earth Campaign. With the Chirr lacking the means to invade any human world anymore and with Republican space forces virtually nonexistent, a stalemate ensues. The ODCs are kept offline, as potential orbital bombardment is still estimated to be capable of destroying the array before the alien fleet is crippled. The Chirr fleet soon moves to neutralize human outposts in the Great Belt, presumably in order to gain a permanent foothold in the Sol system, which allows Earth to physically link up with its colonies in what is another Chirr mistake. The Battle of the Belt ensues between the Chirr colonisation fleet and a loose coalition of human pirate syndicates (which forces number anywhere between 200 and 250 vessels) - it is now the largest space-to-space battle in human history. Most pirate ships are virtually melted away by the vastly technologically superior alien vessels, but their swarming tactics, and sometimes even attempts at ramming, are able to destroy the vast majority of invader forces, leaving only eight Chirr vessels standing. The Chirr abandon their Belt Campaign and return to their blockading positions in Earth's orbit; however, at this point the Orbital Defense Cannon array is turned online, and it soon opens fire, decimating the alien vessels, which are only able to destroy one of the main six ODC Complexes, but lose three ships in the process and get one disabled due to its damages. Finally realising that victory is completely unachievable and facing massive mutinies aboard his own vessels, the commander of the Chirr colonisation fleet orders his ships to turn their weapons offline and attempts to hail the humans asking for peace. Noticing the change in the Chirr vessels' heat signatures and receiving the alien transmission, the ODC batteries are ordered to halt their fire, and after confirming the message's meaning the Republic agrees to a ceasefire. This effectively concludes the Human-Chirr War, which ends in human victory.

The biggest mistake made is they didn't turn one the orbital defense cannons at the start. Second they only made six of them. For best defense of a planet they need at least 100 ODC stations.

7861459
That's true. These definitely ended up being human mistakes in the end, although there are reasons why they acted the way they did.

In hindsight, as you mentioned, the best plan would've probably been to lure the Chirr fleet into Earth's orbit at the very start of the war, and then engage it with both the Navy and the ODCs. The reason this was not done though was mostly because the Republic wanted to protect the way to human colonies as well, hence why the Navy met the Chirr in Mars' orbit, which was naturally well past the ODC effective firing range. They also kinda hoped the Chirr wouldn't turn out hostile, but we know how that went now. In general though, after the space battle in Mars' orbit, the ODCs were kept offline in hopes of concealing their presence and preventing their destruction at the Chirr's hands, as their colonisation fleet was projected to be able to destroy all the main complexes before the ODCs cripple said fleet. Thus, the humans waited for the most appropriate moment to activate the Cannons.

Not having enough ODCs on Earth was arguably an even bigger mistake though. However, the Cannons were mostly built to fend off pirate attacks in the aftermath of the Unification Wars; left unchecked, the pirate gangs and syndicates grew immensely during the conflict, and now posed a significant danger even to Earth itself. At the time, noone really thought the ODCs would need to fend off an attack by a large, organised force, and thus the Republican government on Earth, a planet which was still devastated after the Unification Wars, decided to cut some corners.
The six complexes mentioned here were also just the biggest, main batteries; there were more minor ones, but without assistance from the bigger Cannons these small ones would probably not stand a chance.

And yupp, in modern times when OSaGA takes place (~6200 AD) Earth is probably protected by hundreds of improved ODCs, I'd say.

7861557
That sounds like how we humans always learn. The hard way. I am glad to see you showed he cheep we humans can be in are defense. Always great seeing these updates.