It is entirely possible to win without building any warships, simply by invading the enemy planets. All races can make use of this tactic, but some races excel at this, in particular, the Bulrathi due to their enhanced ground combat skill. Because it works so well with Bulrathi, I call this the Bulrathi Wave. The Sakkra, due to their high population growth and the Psilon, due to their technical prowess, also are very solid with this tactic. The Silicoids, due to their slow population growth are NOT good candidates to use this tactic, but EXCELLENT candidates to use this tactic on.
The Bulrathi Wave[edit]
Once any planet with a colony is scouted any of your planets can send transports to it. If there is an enemy colony there you will be "invading" it. Enemy ships and missile bases will shoot at your transports before they land, but unless the colony is extremely well defended some will get through. This can often be exploited to tremendous effect.
The trick is to make sure 1) Your ground fighting is as strong as possible. Always take technologies that enhance your ground fighting first 2) your population growth is strong. Cloning and advanced cloning help add population quickly 3) Higher propulsion leads to fewer losses.
Some technologies are especially helpful with this tactic. Advanced System Scanner allows you to see (and thus invade!) nearby systems without having to visit them or deal with those pesky missile bases. Combat Transporters ensure half your transports reach the planet (unless they have Interdictors). Cloning and advanced cloning allow you to repopulate quickly. Terraforming allows more population with allows more transports.
Once those are in place the Bulrathi Wave looks like this: all, or most, of your planets each choose a different enemy plant to send as many transports as possible to. Generally, each planet should choose a planet others haven't chosen. Then, you immediately focus that planet on replacing its population. As your transports fly to glorious conquest, your planets are repopulated for the next wave.
If you are at the same technology level as your opponent, but you are Bulrathi and/or have focused on weapons tech then each transport that lands should kill at least 2 and often closer to 5 or more enemies. So, if you have a planet of population 100, you can send half the transports, or 50 to invade. If you are invading a planet of 100 you only need 20 of your 50 to survive to take over the planet. Moreover, if you succeed you will steal a LOT of tech AND get a colony that already has a lot of factories. If you fail you will still likely have severely depleted the planet’s population and thus its industrial output.
The computer likes to mass ships. If they have one planet with a lot of ships, you might avoid that planet and send those transports to another planet.
Let's say you have 5 colonies and your enemy has 5 colonies. They are sending fleets at you that you are having a hard time dealing with. If you've scouted their systems, with ships, spies or Advanced System Scanner, you can respond to this with a Bulrathi Wave. Each of your colonies sends a transport to one of the enemy’s planets. In this case, let's assume one of the enemy planets is amassing ships, so you avoid that one and send those extra transports to a large planet that might need a few more invaders to conquer.
Generally, I will win 2 or 3 of the enemy planets, and by the time the wave gets there the next wave will be ready to go. I will now have all of the enemy technologies, stolen from their factories on invasion, and I can work on winning the remaining few planets. Pretty quickly it goes from an even 5 on 5 match to a 8 to 2 match. If the enemy has a big invasion fleet amassing I'll wait until they launch it at me and then send enough transports from all my colonies to take that planet as well. Instead of the usual slow invasion by sending ships to clear the enemy out, this is higher stakes but often is decisive in just a few turns.
A few caveats: 1) Make sure your fighting technology is ahead of your opponents. This tactic is glorious if you have the advantage. It's devastating if you don't as you simply end up sending waves of population to their deaths. 2) Barely losing a planet is better than barely winning! If you win and have only a few people left alive they will retake the colony and steal a lot of technology from you, likely ruining your ground attack advantage! If they have only a few left they are easy pickings for a second wave and very gimped. If you end up with only a few population on a planet with a lot of factories send reinforcements immediately! 3) It's not unusual for enemies to attack and wipe out these settlements soon after you take them. While not ideal, it's essentially the enemy wiping out their own colony for you.