Yes, Anacreon should have other way to conquer a world beside using military forces to invading it. This method (Diplomatic method, I might say) purpose is spreading empire influence, convince independent world to join the empire without any violence. So without using combat ships, we can still conquer the world via propaganda.

Diplomatic method should only available for non aggression doctrines such as Trade & Enterprise (or in future doctrines, Science & Technology and Growth & Industry). This is makes sense because these doctrines should has friendly way to conquer the worlds.

Non-aggression doctrines still can invading other planet, especially when empire want designate sector capital on outer 250 LY of capital influence. However, this action should severely damages conquered planet's social order. Because people expected these doctrine is as friendly alignment, not as an ambitious expansionist.

This method should adhere these following rules:

1. Diplomacy only uses explorer. The worlds always refuse to negotiate if there a some combat ship and transport in the orbit (even if they are from another player).

2. Targeted world is independent and not a part of other empire. Also should located within 250 LY from imperial capital or sector capital.

3. This method only works on world that has tech level less than or equal to TL6 (Fusion). Beyond TL6, the world will always refuse to join. TL1 worlds has 60% to success; TL2 has 50%; TL3 has 40%; TL4 has 30%; TL5 has 20%; and TL 6 has 10%.

4. At maximum 1000 explorer, the success percentage increased up to 20%. That's mean, player have maximum 80% to successfully obtain the world.

The downside of this method is it takes very long time to conquer. A successful invasion takes around 5 minutes, but campaign takes at maximum a half or one hour and it's not always works. It comes with a big reward however. Empire will have a world with intact defenses. Empire also could obtain all ship that stationed in that planet. That's mean, player might get that 2k GDM defenses (and might get a bunch of starship in that world, if it was designated as shipyard by previous empire) for free! Also social order will start at content.