I have completed my first Diplomacy SP game (which is 1v1 large map) just about, they have 1 planet left but it's not colonised due to superweapon fire (I have been reading that the AI doesn't use them so in future against AI I will not use them either).

It turned out that at the start of this game the enemy had in his home system an artifact that gave him 10% more weapons damage and 15% more range. In one of the contested systems he also gained the antimatter artifact. As a result he was winning the engagements where I had thought I could win, and so I had trouble contesting the unsettled systems against him. I tried to sneak a few ships through to a planet I had located via scouting which he hadn't got to, but he was there before I could make it. His military ended up being way larger than mine and I was limited to two planets and two crystal mines.

This ended up with me having three attack avenues into my home system, a wormhole, the star, and a phase jump from the now enemy held galaxy adjacent. So never having played diplomacy before, I discovered the utility of starbases. In a sector that is otherwise not colonisable I built a starbase that effectively closed off the link from the adjacent galaxy.

He threw fleets at it that were 3-4 capships plus a LOT of carrier cruisers and I did have to rebuild that starbase a few times (I was having trouble affording upgrade tech and sustaining this defense because I had a severe lack of crystal) but I always tried to have a constructor within two jumps ready to go. However between it and my much smaller fleet (I split my ships into 2, a larger 3 capship expeditionary and a smaller high level carrier plus Kol for defence) I normally stopped his assault (some stragglers would make it through to a core world but it was never a problem), the adjacent system would be a lot less dangerous as he had just comitted a great deal of his fleet. The wormhole was a problem too, but I eventually built a starbase near the wormhole and he for some reason basically stopped using the wormhole after that. I did have issues with them coming in from the star, but my worlds whilst not yet starbased were well defended and I would let them get heavily engaged and jump my defensive fleet in behind them.

When the adjacent system was less dangerous due to their failed attacks I managed to do some hit and runs, once an incomplete Orkulus was taken care of, the capture of non colonisable areas with starbases to secure the mines, and eventually go some way to solving my crystal problem. It was slow going, and it was expensive as starbases were assaulted and occasionally destroyed when my fleet could not respond. Eventually, my starbases were somewhat upgraded and it would take a massive effort for them to get past a starbase with my now reasonably large fleet roaming around. But the starbase would never survive if I left it alone. However I had another problem. I had two planets and he way more.

I decided to assault the world where I had destroyed the Orkulus. I had regular scout missions going there and the mines weren't too dense. He had a new Orkulus but I figured I could take care of it as by now my exepeditionary fleet was a level 10 Sova, a high level Kol, a level 10 Marza, two mid level Kols and a mid level Akkan plus a lot of LRFs, flaks, Kodiaks and Hoshikos. How wrong I was. I jumped in and the Orkulus wasn't at the entrance for some reason, but it made it's way over and it started. There was not a lot of enemy fleet to worry about but there was hangar defenses. I had my fighters target enemy strikecraft in general, and assigned bombers to the Orkulus havng never engaged one before. IT WOULD NOT GO DOWN. My ships seemed to just pop. I had to fall back - I ended up with all my caps, several heavily damaged, but next to NO frigates or cruisers left. Why was it so damn strong whereas my own starbases seemed rather fragile?? It was probably the hull upgrades and weapons research I couldn't do due to not having enough slots to build research stations. Why would my ships wither against it whilst they seemed to be able to stand and fight longer? Probably hull research and that weapons/antimatter artifact combo that the AI had acquired that I didn't yet know about. I couldn't flood him with frigates as I just didn't have the fleet capacity. I considered retiring a capship for more frigates but eventually decided against that.

So I went back to securing sectors that were not colonisable and stated sacrificing other facilities at my planets to get research done. I eventually got control of a gravwell that was an important one - a gas giant, that essentially cut off one of his worlds from everywhere else. By holding this sector I pretty much disabled a lot of his local fleet production as he had been making capships there. Low level caps and a few frigates were not able to take care of my starbase with 16 fighter squads. To attack it from the other world the AI owned in this system, he had to traverse gravwells that were starbased. But the issue remained how do I assault his worlds with those damned Orkulus things?

I eventually did the research for the Ogrov. Wait! That's not in standard SOasE either! So the pros are, good against structures, very long range. Cons, Orkulus can move into weapons range against them, no defensive capabilities whatsoever, and the Orkulus is now right on the jump lane so you jump on top of him and can't get the torps in without them taking immediate fire. The solution was to use flaks because they're fairly tough, Jump them in and distract Orky across the gravwell, and then jump in the fleet. This worked a charm as my fleet attacked it with bombers whilst they quickly took out the turrets and hangars which were on the other side of the gravwell. This allowed me to pick my engagement. Once his shields were down the torps came in and he went down under attack from two sides. Thankyou very much - I now had a foothold in another system.

It went from there pretty much. I captured the world that was bottled up by the gas giant as well and could then expand my empire. Due to my securing of resource only sectors, I was now still a bit short of crystal but not to the extent I was. I built up my trade routes, and once starbases were fully upgrdaded I realised why his were so hard to crack.

My only issue after that was losses due to massive minefields but as mentioned in another thread I used flaks and scoutships to thin them out. And once I captured those artifacts, things were a LOT easier.

My next game will expand to a 4 player map with 3 AI unlocked teams.